manual: clarify policy when updating a package patch

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Renaud AUBIN <root@renaud.io>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

docs/manual/patch-policy.txt

@@ -106,6 +106,10 @@ upstream SCM software to generate the patch set.
 Otherwise, concatenate the header with the output of the
 +diff -purN package-version.orig/ package-version/+ command.
 
+If you update an existing patch (e.g. when bumping the package version),
+make sure the existing From header and Signed-off-by tags are not
+removed, but do update the rest of the patch comment when appropriate.
+
 At the end, the patch should look like:
 
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