Fix git-rerere documentation

rerere.enabled is _not_ on by default.  The command is enabled if rr-cache
exists even when rerere.enabled is missing, and enabled or disabled by
explicitly setting the rerere.enabled variable.

Documentation/config.txt

@@ -543,8 +543,8 @@ rerere.enabled::
 	Activate recording of resolved conflicts, so that identical
 	conflict hunks can be resolved automatically, should they
 	be encountered again.  linkgit:git-rerere[1] command is by
-	default enabled, but can be disabled by setting this option to
-	false.
+	default enabled if you create `rr-cache` directory under
+	`$GIT_DIR`, but can be disabled by setting this option to false.
 
 gitcvs.enabled::
 	Whether the CVS server interface is enabled for this repository.

Documentation/git-rerere.txt

@@ -22,6 +22,10 @@ automerge results and corresponding hand-resolve results on the
 initial manual merge, and later by noticing the same automerge
 results and applying the previously recorded hand resolution.
 
+[NOTE]
+You need to set the configuration variable rerere.enabled to
+enable this command.
+
 
 COMMANDS
 --------

Makefile

@@ -1187,3 +1187,4 @@ check-docs::
 #
 check-builtins::
 	./check-builtins.sh
+