[Tiny] Add allowed characters for custom BBCodes to ACP page

ACP -> Posting -> BBCodes -> Add/Edit BBCode:

It would be good if the characters which are allowed for custom BBCodes were listed on this page.

Maybe you can append them to this sentence:
language/en/acp/posting.php - BBCODE_USAGE_EXPLAIN:

Here you define how to use the BBCode. Replace any variable input by the corresponding token (see below).