bug#39778: [PATCH] checkdoc: Don't mistake "cf." for sentence end

[Drew Adams]

> "Cf." is a common abbreviation in documentation.
> 
> Currently checkdoc warns about lack of double space after "cf." or
> "Cf.". The attached patch adds an exception similar to the already
> handled "eg." and and "ie.".

Yes.

But even better is to remove uses of "Cf." (which
is not so common nowadays, and is understood by
fewer and fewer readers).  It can typically be
replaced by something as simple as "Compare" or
sometimes even just "See also".