Update and clarify ssl_ciphers default

- Write HIGH:MEDIUM instead of DEFAULT:!LOW:!EXP for clarity.
- Order 3DES last to work around inappropriate OpenSSL default.
- Remove !MD5 and @STRENGTH, because they are irrelevant.
- Add clarifying documentation.

Effectively, the new default is almost the same as the old one, but it
is arguably easier to understand and modify.

Author: Marko Kreen <markokr@gmail.com>

src/backend/utils/misc/guc.c

@@ -3237,7 +3237,7 @@ static struct config_string ConfigureNamesString[] =
 		},
 		&SSLCipherSuites,
 #ifdef USE_SSL
-		"DEFAULT:!LOW:!EXP:!MD5:@STRENGTH",
+		"HIGH:MEDIUM:+3DES:!aNULL",
 #else
 		"none",
 #endif