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>