Document "git-reset --merge"

The commit log message for the feature made it sound as if this is a saner
version of --mixed, but the use case presented makes it clear that it is a
better variant of --hard when your changes and somebody else's changes are
mixed together.

Perhaps we would want to rewrite the example that shows the use of --hard
not to talk about recovering from a failed merge?

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-reset.txt

@@ -8,7 +8,7 @@ git-reset - Reset current HEAD to the specified state
 SYNOPSIS
 --------
 [verse]
-'git reset' [--mixed | --soft | --hard] [-q] [<commit>]
+'git reset' [--mixed | --soft | --hard | --merge] [-q] [<commit>]
 'git reset' [-q] [<commit>] [--] <paths>...
 
 DESCRIPTION
@@ -45,6 +45,11 @@ OPTIONS
 	switched to. Any changes to tracked files in the working tree
 	since <commit> are lost.
 
+--merge::
+	Resets the index to match the tree recorded by the named commit,
+	and updates the files that are different between the named commit
+	and the current commit in the working tree.
+
 -q::
 	Be quiet, only report errors.
 
@@ -152,6 +157,28 @@ tip of the current branch in ORIG_HEAD, so resetting hard to it
 brings your index file and the working tree back to that state,
 and resets the tip of the branch to that commit.
 
+Undo a merge or pull inside a dirty work tree::
++
+------------
+$ git pull                         <1>
+Auto-merging nitfol
+Merge made by recursive.
+ nitfol                |   20 +++++----
+ ...
+$ git reset --merge ORIG_HEAD      <2>
+------------
++
+<1> Even if you may have local modifications in your
+working tree, you can safely say "git pull" when you know
+that the change in the other branch does not overlap with
+them.
+<2> After inspecting the result of the merge, you may find
+that the change in the other branch is unsatisfactory.  Running
+"git reset --hard ORIG_HEAD" will let you go back to where you
+were, but it will discard your local changes, which you do not
+want.  "git reset --merge" keeps your local changes.
+
+
 Interrupted workflow::
 +
 Suppose you are interrupted by an urgent fix request while you