curl_easy_setopt.3: proto updates + cleanups

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.