Add ciphers doc

doc/man1/ciphers.pod

@@ -405,6 +405,21 @@ permissible.
 
 =back
 
+=head1 EQUAL PREFERENCE GROUPS
+
+If configuring a server, one may also configure equal-preference groups to
+partially respect the client's preferences when
+B<SSL_OP_CIPHER_SERVER_PREFERENCE> is enabled. Ciphers in an equal-preference
+group have equal priority and use the client order. This may be used to
+enforce that AEADs are preferred but select AES-GCM vs. ChaCha20-Poly1305
+based on client preferences. An equal-preference is specified with square
+brackets, combining multiple selectors separated by |. For example:
+
+ [ECDHE-ECDSA-CHACHA20-POLY1305|ECDHE-ECDSA-AES128-GCM-SHA256]
+ 
+ Once an equal-preference group is used, future directives must be
+ opcode-less.
+
 =head1 CIPHER SUITE NAMES
 
 The following lists give the SSL or TLS cipher suites names from the