docs/manual: patch subjects shall not be numbered

The script "utils/check-package" checks that patch email prefix are
not be numbered. See:
https://git.buildroot.org/buildroot/tree/utils/checkpackagelib/lib_patch.py?h=2022.08-rc1#n42

The error message recommends to generate patches to be included in
Buildroot with the command 'git format-patch -N'.

The patch policy section in the Buildroot manual does mention that.

This commit adds a note about that requirement.

Signed-off-by: Julien Olivain <ju.o@free.fr>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

docs/manual/patch-policy.txt

@@ -44,6 +44,11 @@ These patch files should be named +<number>-<description>.patch+.
 - The field +<number>+ in the patch file name refers to the 'apply order',
   and shall start at 1; It is preferred to pad the number with zeros up to 4
   digits, like 'git-format-patch' does. E.g.: +0001-foobar-the-buz.patch+
+- The patch email subject prefix shall not be numbered. Patches shall
+  be generated with the +git format-patch -N+ command, since this
+  numbering is automatically added for series. For example, the patch
+  subject line should look like +Subject: [PATCH] foobar the buz+ rather
+  than +Subject: [PATCH n/m] foobar the buz+.
 - Previously, it was mandatory for patches to be prefixed with the name of
   the package, like +<package>-<number>-<description>.patch+, but that is
   no longer the case. Existing packages will be fixed as time passes. 'Do