Documentation: rerere's rr-cache auto-creation and rerere.enabled

The description of rerere.enabled left the user in the dark as to who
might create an rr-cache directory.  Add a note that simply invoking
rerere does this.

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

Documentation/config.txt

@@ -1703,7 +1703,8 @@ rerere.enabled::
 	conflict hunks can be resolved automatically, should they be
 	encountered again.  By default, linkgit:git-rerere[1] is
 	enabled if there is an `rr-cache` directory under the
-	`$GIT_DIR`.
+	`$GIT_DIR`, e.g. if "rerere" was previously used in the
+	repository.
 
 sendemail.identity::
 	A configuration identity. When given, causes values in the