doc/man3/EVP_PKEY_CTX_ctrl.pod: cleanup

Clean up a manual we've touched, according to conventions found in
Linux' man-pages(7); function arguments in descriptions should be in
italics, and types, macros and similar should be in bold, with the
exception for NULL.
Reviewed-by: NShane Lontis <shane.lontis@oracle.com>
Reviewed-by: NTomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/10557)