DOC: add examples and extend existing dos for polynomial subclasses

[jeffyancey]

Documentation changes from #11811

[jeffyancey]

@charris and @eric-wieser, should the pow fncs docstrings be modified to explicitly state that that array_like parameter must not be empty? Or should these functions be modified to support empty arrays?

Currently the polyutils.as_series() is used, which causes the follow error from line 176 if the array is empty:

ValueError: Coefficient array is empty

[eric-wieser]

I don't think it needs to - if anything, I'd prefer to make polyutils.as_series work for empty arrays

[eric-wieser]

Small typo in commit message, but I can fix it when I merge

[charris]

Thanks @jeffyancey .