Documentation: mention conflict marker size argument (%L) for merge driver

23a64c9e (conflict-marker-size: new attribute, 2010-01-16) introduced the
new attribute and also pass the conflict marker size as %L to merge driver
commands. This documents the substitution.
Signed-off-by: NBert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/gitattributes.txt

@@ -511,7 +511,8 @@ command to run to merge ancestor's version (`%O`), current
 version (`%A`) and the other branches' version (`%B`).  These
 three tokens are replaced with the names of temporary files that
 hold the contents of these versions when the command line is
-built.
+built. Additionally, %L will be replaced with the conflict marker
+size (see below).
 
 The merge driver is expected to leave the result of the merge in
 the file named with `%A` by overwriting it, and exit with zero