Improve documentation for word (over str vs over integer)

[slabbe]

As noticed by Florent Hivert ​here, the following two objects :

sage: w = Word('010120')
sage: z = Word([0, 1, 0, 1, 2, 0])

have the same string representation :

sage: w
word: 010120
sage: z
word: 010120

but are not equal :

sage: w == z
False

Indeed, w and z are defined on different alphabet :

sage: w[2]
'0'
sage: z[2]
0

Documentation about this should be improved.

Also that url looks very bad: ​http://doc.sagemath.org/html/en/reference/combinat/sage/combinat/words/__init__.html

which is referenced from ​http://doc.sagemath.org/html/en/reference/combinat/index.html

[slabbe]

I am cleaning my old trac tickets during Sage Days 75.

[slabbe]

New commits:

​fe28a55	10261: cleaning documentation of main page of combinat/words in reference manual
​4c5e509	10261: documenting ambiguous representation of words

[git]

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

​68d1a03	10261: cleaning documentation of main page of combinat/words in reference manual
​97dada0	10261: documenting ambiguous representation of words

[tmonteil]

Through this way of indexing the contents of the doc via __init__.py files is specific to the combinat section (which leads to weird URLs), the commits fix the issue and the doc is improved.