doc: clarify rerere-autoupdate

The "--[no-]rerere-autoupdate" option controls what happens _after_
the rerere mechanism kicks in to reuse recorded resolutions and does
not prevent from the rerere mechanism to trigger in the first place.

It is unclear in the current text if "--no-rerere-autoupdate" stops
the auto-resolution.  Rewrite the sentence to clarify.

Signed-off-by: Junio C Hamano <[email protected]>

Author: gitster

Documentation/rerere-options.txt

@@ -1,4 +1,9 @@
 --rerere-autoupdate::
 --no-rerere-autoupdate::
-	Allow the rerere mechanism to update the index with the
-	result of auto-conflict resolution if possible.
+	After the rerere mechanism reuses a recorded resolution on
+	the current conflict to update the files in the working
+	tree, allow it to also update the index with the result of
+	resolution.  `--no-rerere-autoupdate` is a good way to
+	double-check what `rerere` did and catch potential
+	mismerges, before committing the result to the index with a
+	separate `git add`.