Document the logx parameter in Expr.nseries()

Add doctests. This parameter was introduced in
9c46974da12df56818020e29f76313fc0903ecc6 but it was never documented.

Question: I still don't understand what logx is doing. For majority of cases, you can simply substitute for log(x) before you even start expanding. I put there one example that I figured out, where you cannot substitute before you start expanding. Can somebody provide even better example and more importantly --- explain what exactly the logx is doing?

/cc @ness01