Documentation: improve description of GIT_EDITOR and preference order

Documentation: improve description of GIT_EDITOR and preference order

Previously GIT_EDITOR was not listed in git(1) "Environment Variables" section,
which could be very confusing to users. Include it in "other" subsection along
with a link to git-var(1), since that is the page that fully describes all
places where editor can be set and also their preference order.

Also, git-var(1) did not say that hardcoded fallback 'vi' may have been changed
at build time. A user could be puzzled if 'nano' pops up even when none of the
mentioned environment vars or config.editor are set. Clarify this.

Ideally, the build system should be changed to reflect the chosen fallback
editor when creating the man pages. Not sure if that is even possible though.

Signed-off-by: Rodrigo Silva (MestreLion) <linux@rodrigosilva.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>