Update git-merge documentation.

Update git-merge documentation.

Add "Configuration" section to describe merge.summary
configuration variable (which is mentioned in git-fmt-merge-msg(1)
man page, but it is a plumbing command), and merge.verbosity
configuration variable (so there is a place to make reference
from "Environment Variables" section of git(7) man page) to the
git-merge(1) man page.  Also describe GIT_MERGE_VERBOSITY
environment.

The configuration variable merge.verbosity and environment variable
GIT_MERGE_VERBOSITY were introduced in commit 8c3275ab, which also
documented configuration variable but not environment variable.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>