Note which ssl config vars are GnuTLS or OpenSSL only. (closes #3781)

It was not obvious that the ssl_ca_certificates_file was only for
GnuTLS, or that the ssl_usesystemcerts was only for OpenSSL.

init.h

@@ -3394,7 +3394,7 @@ struct option_t MuttVars[] = {
   ** .pp
   ** This variable specifies a file containing trusted CA certificates.
   ** Any server certificate that is signed with one of these CA
-  ** certificates is also automatically accepted.
+  ** certificates is also automatically accepted. (GnuTLS only)
   ** .pp
   ** Example:
   ** .ts
@@ -3423,7 +3423,7 @@ struct option_t MuttVars[] = {
   ** .pp
   ** This variable specifies the minimum acceptable prime size (in bits)
   ** for use in any Diffie-Hellman key exchange. A value of 0 will use
-  ** the default from the GNUTLS library.
+  ** the default from the GNUTLS library. (GnuTLS only)
   */
 # endif /* USE_SSL_GNUTLS */
   { "ssl_starttls", DT_QUAD, R_NONE, OPT_SSLSTARTTLS, MUTT_YES },
@@ -3440,6 +3440,7 @@ struct option_t MuttVars[] = {
   ** This variable specifies whether to attempt to use SSLv2 in the
   ** SSL authentication process. Note that SSLv2 and SSLv3 are now
   ** considered fundamentally insecure and are no longer recommended.
+  ** (OpenSSL only)
   */
 # endif /* defined USE_SSL_OPENSSL */
   { "ssl_use_sslv3", DT_BOOL, R_NONE, OPTSSLV3, 0 },
@@ -3473,7 +3474,7 @@ struct option_t MuttVars[] = {
   ** .pp
   ** If set to \fIyes\fP, mutt will use CA certificates in the
   ** system-wide certificate store when checking if a server certificate
-  ** is signed by a trusted CA.
+  ** is signed by a trusted CA. (OpenSSL only)
   */
 #endif
   { "ssl_verify_dates", DT_BOOL, R_NONE, OPTSSLVERIFYDATES, 1 },