Documentation/git-merge-file: document option "--diff3"

The option "--diff3" was added to "git merge-file" in e0af48e
(xdiff-merge: optionally show conflicts in "diff3 -m" style)
but it was never documented in "Documentation/git-merge-file.txt".
Add documentation for this option.

Signed-off-by: Ralf Thielow <ralf.thielow@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-merge-file.txt

@@ -11,7 +11,7 @@ SYNOPSIS
 [verse]
 'git merge-file' [-L <current-name> [-L <base-name> [-L <other-name>]]]
 	[--ours|--theirs|--union] [-p|--stdout] [-q|--quiet] [--marker-size=<n>]
-	<current-file> <base-file> <other-file>
+	[--[no-]diff3] <current-file> <base-file> <other-file>
 
 
 DESCRIPTION
@@ -66,6 +66,9 @@ OPTIONS
 -q::
 	Quiet; do not warn about conflicts.
 
+--diff3::
+	Show conflicts in "diff3" style.
+
 --ours::
 --theirs::
 --union::