grep: Move lone 'r'grep case into the adjacent switch

This 'r' case should have belonged to the switch in the first place, but
I had somehow missed the switch when initially adding the rgrep link. The
zgrep script later came along and faithfully left this case standing alone,
so we will now go ahead and join it.

Nearby comment also adjusted a tad bit for wording and style.

Reported by: Daniel Ebdrup
MFC after: 3 days