Log Message:
guide: Improve/extend documentation about FTP layout

 - Document the `current', `stable' and `pkgsrc-20xxQy' more extensively,
   from J. Lewis Muir via PR pkg/54019.  Thanks!
 - Refers to `pkgsrc.tar.{bz,gz,xz}' instead of just `pkgsrc.tar.gz' to
   cover all compressed formats.
 - Document `pkgsrc-readmes.tar.{bz,gz,xz}'