Documentation/notes: simplify treatment of default display refs

Documentation/notes: simplify treatment of default display refs

The main description of display refs for notes should be in
git-log.1, where there is a chance to give a leisurely description
of all the ways they can be set, what they are used for, and so
on.  The description in git-notes.1 is only meant to be a quick
reminder of how notes are used.

So simplify it.

Also add an entry for GIT_NOTES_DISPLAY_REF to the environment
section.

Cc: Thomas Rast <trast@student.ethz.ch>
Cc: Johan Herland <johan@herland.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>