Document hook_privatemsg_name_lookup()

This is an interesting small glitch, even it don't will appear for the "normal" drupal user.
Thats why i set it to minor.

I am uneasy its worth a code fix, perhaps a note in the readme would be enough.

Let me explain what we found:

At www.daimonin.org we imported an already existing community from another CMS/forum to drupal.

The format of user names was there different, so i had to change the names a bit.
I also don't want gigantic user names, it will break the web 2.0 like site.

So i patched the user length in user.module to 15 bytes.
Means new user can't register a name longer as 15 bytes.

Now, we had old names from the old CMS longer as 15 in the DB. Drupal itself has no problem with it.

But as one of this now "to long" (18 bytes) guys tried to PM me, i was not able to answer his PM.
The user was able to PM other people but when you tried to PM him, the PM module tells you
"you need one recipent to answer".

Also a direct PM don't works, the module is removing him from the recipents list and telling you its not there.
If you use a multi recipents PM, only the valid users get PMed.

Not sure how this is to handle, i patched drupal so the general user length is big enough but new users still can't
register longer names as 15 bytes (and i also think this kind of settings should be part of the core user admin settings).