docs/manual: do not expose CPE_ID_VALID as package-settable

docs/manual: do not expose CPE_ID_VALID as package-settable

FOO_CPE_ID_VALID is an internal implementation detail. Packages should
really define an actual CPE_ID variable to trigger their full CPE_ID
definition.

Reported-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Matthew Weber <matthew.weber@rockwellcollins.com>
Cc: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[Peter: completely drop any mention of _CPE_ID_VALID in the manual]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>