Reflect special `DEFAULT` behavior in ciphers(1)

Actual behavior of DEFAULT is different than currently described.
Rather than actinf as cipher string, DEFAULT cannot be combined using
logical operators, etc.

Fixes #5420.
Reviewed-by: NRich Salz <rsalz@openssl.org>
Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5428)

doc/man1/ciphers.pod

@@ -168,19 +168,20 @@ The cipher string B<@SECLEVEL=n> can be used at any point to set the security
 level to B<n>, which should be a number between zero and five, inclusive.
 See L<SSL_CTX_set_security_level> for a description of what each level means.
 
+The cipher list can be prefixed with the B<DEFAULT> keyword, which enables
+the default cipher list as defined below.  Unlike cipher strings,
+this prefix may not be combined with other strings using B<+> character.
+For example, B<DEFAULT+DES> is not valid.
+
+The content of the default list is determined at compile time and normally
+corresponds to B<ALL:!COMPLEMENTOFDEFAULT:!eNULL>.
+
 =head1 CIPHER STRINGS
 
 The following is a list of all permitted cipher strings and their meanings.
 
 =over 4
 
-=item B<DEFAULT>
-
-The default cipher list.
-This is determined at compile time and is normally
-B<ALL:!COMPLEMENTOFDEFAULT:!eNULL>.
-When used, this must be the first cipherstring specified.
-
 =item B<COMPLEMENTOFDEFAULT>
 
 The ciphers included in B<ALL>, but not enabled by default. Currently