TYP and DOC: pd.merge accepts Series
I noticed that the docs for `pd.merge()` and `pd.merge_asof()` were requiring the first argument to be a `DataFrame`, but in fact, they can be a `Series`.  This PR updates the docs and the types to fix that issue.

Didn't think a `whatsnew` entry was needed, and I skipped creating an issue, but will do so if required.

> I think you should use DataFrame | Series instead of FrameOrSeriesUnion. (See #40876 for more context)

Thanks for pointing that out.  Have now changed it. cc @simonjayhawkins if any comments. Thanks @Dr-Irv 