clarify manual on how to use authinfo

The manual says:

   If your environment has the `EasyPG' utility and your version of
   Emacs supports it, i.e., has the `epa-file' and `auth-source'
   libraries, then you can store password information in a file such
   as `.authinfo.gpg'. The `EasyPG' protocol allows you to store this
   information in an encrypted form so that it cannot be read by third
   parties. Each line in the `.authinfo.gpg' file should be of the
   form...

This might give the impression that this happens automatically,
perhaps would be useful to mention that one needs to (require
'auth-source)? Or perhaps I just missed it...