Doc fixes

Write missing prime.pod and srp.pod
Implement -c in find-doc-nits (for command options)
Other fixes to some manpages
Use B<-I<digest|cipher>> notation
Split up multiple flags into a single entry in the synopsis.
Add -1 and missing-help to list command.

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4144)