improve GETDEL description (redis#1508)

Author: oranagra

commands/getdel.md

@@ -1,9 +1,9 @@
 Get the value of `key` and delete the key.
-This command is identical to `GET`, except for the fact that it also deletes the key.
+This command is similar to `GET`, except for the fact that it also deletes the key on success (if and only if the key's value type is a string).
 
 @return
 
-@bulk-string-reply: the value of `key`, or `nil` when `key` does not exist.
+@bulk-string-reply: the value of `key`, `nil` when `key` does not exist, or an error if the key's value type isn't a string.
 
 @examples