Re: libm man page fixes

While looking at this diff I noticed that we have the following wording:

The
.Fn sin
family of functions return the sine value.

We currently have both versions ("returns" and "return") in different
man pages.

Grammarwise I would say "family" is singular and "returns" is the
singular-form.

On the other hand, it sounds weird (to me at least). Perhaps we should
just reword the whole sentence. Suggestions?
 Thomas