Add use tips to r interface documentation

[charpent]

Inspired by ​this ask.sagemath question.

Add a couple of usage tips to the "r?" help text, with a couple examples.

[charpent]

Added a few usage tips and a few (commented) examples to the help text displayed by r.<tab>

Passes ptestlong with no error (transient or not).

==>needs_review

---

New commits:

​1034a26

Add a few usage tips and examples to r.<tab> help text.

[vdelecroix]

You wrongly used Python commnents (starting with #) inside doctests. The following is allowed

    sage: # my comment
    sage: print(1)
    1

the following is better

My comment::

    sage: print(1)
    1

But the following is wrong

# my comment
sage: print(1)
1

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​e22ab46

23980 : better presentation of the tips' examples.

[charpent]

Replying to git:

Branch pushed to git repo; I updated commit sha1. New commits:

​e22ab46

23980 : better presentation of the tips' examples.

Thanks for your remarks, Vincent ! Sphinx is a bitch for me..

The pushed version looks good on a terminal and in html, passes ptestlong with one failure :

----------------------------------------------------------------------
sage -t --long src/sage/coding/linear_code.py  # Timed out
----------------------------------------------------------------------

which turns out to be transient (the doctest passes with no error when ran standalone).

==>needs_review.

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​7f4602e

23980 : minor typo

[charpent]

Replying to git:

Branch pushed to git repo; I updated commit sha1. New commits:

​7f4602e

23980 : minor typo

Cosmetic update to an example, better look on HTML.

Still needs_review

[vdelecroix]

Your second line in the example fails for me

sage: tt = r.t_test(r.as_formula("y~x"), data=d)
/opt/sage/local/lib/python3.7/site-packages/rpy2/rinterface/__init__.py:185: RRuntimeWarning: Error in strsplit(x, "\n[ \t\n]*\n", perl = TRUE) : motif de découpage '
[ 	
]*
' incorrect

  warnings.warn(x, RRuntimeWarning)
/opt/sage/local/lib/python3.7/site-packages/rpy2/rinterface/__init__.py:185: RRuntimeWarning: De plus : 
  warnings.warn(x, RRuntimeWarning)
/opt/sage/local/lib/python3.7/site-packages/rpy2/rinterface/__init__.py:185: RRuntimeWarning: Warning message:

  warnings.warn(x, RRuntimeWarning)
/opt/sage/local/lib/python3.7/site-packages/rpy2/rinterface/__init__.py:185: RRuntimeWarning: In strsplit(x, "\n[ \t\n]*\n", perl = TRUE) :
  warnings.warn(x, RRuntimeWarning)
/opt/sage/local/lib/python3.7/site-packages/rpy2/rinterface/__init__.py:185: RRuntimeWarning: 
 
  warnings.warn(x, RRuntimeWarning)
/opt/sage/local/lib/python3.7/site-packages/rpy2/rinterface/__init__.py:185: RRuntimeWarning:  Erreur de compilation du motif PCRE
	'this version of PCRE is compiled without UTF support'
	at '
[ 	
]*
'

  warnings.warn(x, RRuntimeWarning)
---------------------------------------------------------------------------
RRuntimeError                             Traceback (most recent call last)
<ipython-input-21-2d3c0885b12f> in <module>()
----> 1 tt = r.t_test(r.as_formula("y~x"), data=d)

/opt/sage/local/lib/python3.7/site-packages/sage/interfaces/r.py in __call__(self, *args, **kwds)
   2003             [1] 3
   2004         """
-> 2005         return self._parent.function_call(self._name, args=list(args), kwds=kwds)
   2006 
   2007 def is_RElement(x):

/opt/sage/local/lib/python3.7/site-packages/sage/interfaces/r.py in function_call(self, function, args, kwds)
   1080         self._check_valid_function_name(function)
   1081         return self.new("%s(%s)"%(function, ",".join([s.name() for s in args] +
-> 1082                                                      [self._sage_to_r_name(key)+'='+kwds[key].name() for key in kwds ] )))
   1083 
   1084     def call(self, function_name, *args, **kwds):

/opt/sage/local/lib/python3.7/site-packages/sage/interfaces/interface.py in new(self, code)
    368 
    369     def new(self, code):
--> 370         return self(code)
    371 
    372     ###################################################################

/opt/sage/local/lib/python3.7/site-packages/sage/interfaces/interface.py in __call__(self, x, name)
    294 
    295         if isinstance(x, string_types):
--> 296             return cls(self, x, name=name)
    297         try:
    298             # Special methods do not and should not have an option to

/opt/sage/local/lib/python3.7/site-packages/sage/interfaces/interface.py in __init__(self, parent, value, is_name, name)
    716         else:
    717             try:
--> 718                 self._name = parent._create(value, name=name)
    719             except (TypeError, RuntimeError, ValueError) as x:
    720                 raise TypeError(x)

/opt/sage/local/lib/python3.7/site-packages/sage/interfaces/interface.py in _create(self, value, name)
    499     def _create(self, value, name=None):
    500         name = self._next_var_name() if name is None else name
--> 501         self.set(name, value)
    502         return name
    503 

/opt/sage/local/lib/python3.7/site-packages/sage/interfaces/r.py in set(self, var, value)
   1126         """
   1127         cmd = '%s <- %s'%(var,value)
-> 1128         out = self.eval(cmd)
   1129 
   1130     def get(self, var):

/opt/sage/local/lib/python3.7/site-packages/sage/interfaces/r.py in eval(self, code, *args, **kwds)
   1339         """
   1340         self._lazy_init()
-> 1341         return str(robjects.r(code)).rstrip()
   1342 
   1343 

/opt/sage/local/lib/python3.7/site-packages/rpy2/robjects/robject.py in __str__(self)
    106                 s.append(x)
    107             rpy2.rinterface.set_writeconsole_regular(f)
--> 108         self.__show(self)
    109         if sys.platform == 'win32':
    110             self.__sink()

RRuntimeError: Error in strsplit(x, "\n[ \t\n]*\n", perl = TRUE) : motif de découpage '

[mkoeppe]

Setting new milestone based on a cursory review of ticket status, priority, and last modification date.