Document the logx parameter in Expr.nseries()

Add doctests. This parameter was introduced in
9c46974da12df56818020e29f76313fc0903ecc6 but it was never documented.
Simplify the last docstring example
Merge pull request #6 from skirpichev/certik-logx

Simplify the last docstring example
Return infinty for factorial(infinity)
Merge pull request #2611 from hargup/factorial

Return infinty for factorial(infinity)
Better explanation of the logx parameter

Also the doctests were polished.
Merge pull request #2444 from certik/logx

Document the logx parameter in Expr.nseries()