[PATCH] Fix documentation for key_extras authentication helper parameter

Hi all,

I am attaching a patch which try to add some notes which is missing from
 key_extras documentation in squid.conf.documented. For example the
current squid.conf.documented in trunk does not say that the
expanded key_extras value is added to the Squid credentials cache and,
hence, will affect authentication.

Please review that the added documentation cover most of the problems
will may appear by key_extras misuse.

Regards,
   Christos