Missing or incomplete docstrings for various ring constructors

[fingolfin]

• free_associative_algebra has no docstring
• @free_associative_algebra has no docstring
• laurent_polynomial_ring docstring for multivariate case has no examples
• @laurent_polynomial_ring has no docstring
• power_series_ring docstring for univariate case has no examples
• power_series_ring docstring for multivariate case is missing
• @power_series_ring has no docstring
• rational_function_field has no docstring
• @rational_function_field has no docstring

Note that in several cases, text and examples suitable for a docstring can be found in the doc/*/*.md files and could be extracted from there (and replaced with a @docs block in the .md file).

In any case, I would recommend to focus on one of these (e.g. free_associative_algebra), and write good docstrings for it; then hopefully those can be copied to the other cases with minor adaptions.

Actually I guess one could use polynomial_ring as that starting point, but I am not sure the docstrings for it are great yet, so perhaps first improve them; I'd also argue it has too many docstrings, some of which should perhaps be consolidated, but that's a matter of taste.