Doc format-patch: clarify --notes use case

Remove double negative, and include the repeat usage across
versions of a patch series.

Signed-off-by: Philip Oakley <philipoakley@iee.org>
Signed-off-by: Jeff King <peff@peff.net>

Documentation/git-format-patch.txt

@@ -196,13 +196,12 @@ will want to ensure that threading is disabled for `git send-email`.
 	after the three-dash line.
 +
 The expected use case of this is to write supporting explanation for
-the commit that does not belong to the commit log message proper
-when (or after) you create the commit, and include it in your patch
-submission.  But if you can plan ahead and write it down, there may
-not be a good reason not to write it in your commit message, and if
-you can't, you can always edit the output of format-patch before
-sending it out, so the practical value of this option is somewhat
-dubious, unless your workflow is broken.
+the commit that does not belong to the commit log message proper,
+and include it with the patch submission. While one can simply write
+these explanations after `format-patch` has run but before sending,
+keeping them as git notes allows them to be maintained between versions
+of the patch series (but see the discussion of the `notes.rewrite`
+configuration options in linkgit:git-notes[1] to use this workflow).
 
 --[no]-signature=<signature>::
 	Add a signature to each message produced. Per RFC 3676 the signature