Better docs for Subs.

- Added examples to resolve confusion regarding the result of subs and the limiting value of the expression at that point
- Added examples to clearly show the difference between substitution and numerical evaluation.
- Added `evalf` in the `See Also` of `Subs` documentation section.
edit fancysets docstrings
Merge pull request #2649 from hargup/fix_subs_doc

Better docs for Subs.
Merge pull request #2674 from smichr/fancy

edit fancysets docstrings
merge with master