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) <firstname.lastname@example.org> Signed-off-by: Junio C Hamano <email@example.com>