doc/snac8: elaborate regex blocking

It seems regex blocking only works with lower case letters, since the case will be ignored anyway this is fine. This updates the documentation to express this.

doc/snac.8

@@ -329,8 +329,9 @@ These weapons of mass destruction can be written into the
 file in the server base directory, one per line; if this file exists,
 all posts' content will be matched (after being stripped of HTML tags)
 against these regexes, one by one, and any match will make the post to
-be rejected. If you don't know about regular expressions, don't use this
-option (or learn about them in some tutorial, there are gazillions of
+be rejected. Use lower case, the regex will be case insensitive by default. 
+If you don't know about regular expressions, don't use this
+option (or learn about them inw some tutorial, there are gazillions of
 them out there), as you and your users may start missing posts. Also,
 given that every regular expression implementation supports a different
 set of features, consider reading the documentation about the one