RealBall: Document input aliasing

[cheuberg]

Add a warning that identical balls are assumed to represent the same number.

[cheuberg]

As discussed in ​#17194, input aliasing should be documented.

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Last 10 new commits:

​aa39c10	#17194 RealBall: abstract away access to parent precision
​d3e3998	#17194 RBF: add a precision() method
​ce51f81	#17194 RBF: some more is_foo() and similar methods
​1d92acf	Trac #17194: remove fmpr.pxd (partial revert of dd8933a)
​5e41fcb	Trac #17194: remove imports of fmpr
​4e7ec46	Trac #17194: is_zero, is_nonzero, is_exact
​9e26152	Trac #17194: Implement comparisons
​1196b57	Trac #17194: Fix RealBallField._an_element_
​195a6ee	Trac #17194: Fix two broken references
​a445cf9	Trac #17809: Document input aliasing

[fredrik.johansson]

Is it obvious that "identical real balls" means the same Sage object, not different objects with identical values?

I guess the example should be z = x * x, not z = x^2, as the latter would be correct regardless.

[git]

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

​178f31e

Trac #17809: x*x instead of x^2; mention RealBall objects

[cheuberg]

Replying to fredrik.johansson:

Is it obvious that "identical real balls" means the same Sage object, not different objects with identical values?

I hope that it is clearer now.

I guess the example should be z = x * x, not z = x^2, as the latter would be correct regardless.

Of course, thank you, fixed.