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 8c3275a, 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>

Documentation/config.txt

@@ -567,6 +567,7 @@ merge.verbosity::
 	message if conflicts were detected. Level 1 outputs only
 	conflicts, 2 outputs conflicts and file changes.  Level 5 and
 	above outputs debugging information.  The default is level 2.
+	Can be overriden by 'GIT_MERGE_VERBOSITY' environment variable.
 
 merge.<driver>.name::
 	Defines a human readable name for a custom low-level

Documentation/git-merge.txt

@@ -43,6 +43,21 @@ If you tried a merge which resulted in a complex conflicts and
 would want to start over, you can recover with
 gitlink:git-reset[1].
 
+CONFIGURATION
+-------------
+
+merge.summary::
+	Whether to include summaries of merged commits in newly
+	created merge commit. False by default.
+
+merge.verbosity::
+	Controls the amount of output shown by the recursive merge
+	strategy.  Level 0 outputs nothing except a final error
+	message if conflicts were detected. Level 1 outputs only
+	conflicts, 2 outputs conflicts and file changes.  Level 5 and
+	above outputs debugging information.  The default is level 2.
+	Can be overriden by 'GIT_MERGE_VERBOSITY' environment variable.
+
 
 HOW MERGE WORKS
 ---------------

Documentation/git.txt

@@ -411,6 +411,11 @@ parameter, <path>.
 
 other
 ~~~~~
+'GIT_MERGE_VERBOSITY'::
+	A number controlling the amount of output shown by
+	the recursive merge strategy.  Overrides merge.verbosity.
+	See gitlink:git-merge[1]
+
 'GIT_PAGER'::
 	This environment variable overrides `$PAGER`.