curl_easy_setopt.3: proto updates + cleanups
- For all *FUNCTION options, they now all show the complete prototype in
the description. Previously some of them would just refer to a
typedef'ed function pointer in the curl.h header.
- I made the phrasing of that "Pass a pointer to a function that matches
the following prototype" the same for all *FUNCTION option descriptions.
- I removed some uses of 'should'. I think I sometimes over-use this
word as in many places I actually mean MUST or otherwise more specific
and not-so-optional synonyms.