Fix documentation
:math: is changed to .. math ::
Move generic method taylor_term to Expr class
Make Pow.taylor_term a helper function to handle just (1+x)**e case
Add test for issue 3330
Merge pull request #2347 from skirpichev/3330-pow-taylor_term

Fix issue 3330
Merge pull request #2295 from idlike2dream/erf_doc

Fix documentation of error functions