Use flags for raspberrypi-kernel package need documentation

[jonesmz]

E.g. the below are in need of fixing.

GenPi64 ~ # equery u raspberrypi-kernel
[ Legend : U - final flag setting for installation]
[        : I - package is installed with flag     ]
[ Colors : set, unset                             ]
 * Found these USE flags for sys-kernel/raspberrypi-kernel-5.10.90:
 U I
 - - bcm2709     : <unknown>
 + - bcm2711     : <unknown>
 - - bcmrpi      : <unknown>
 + - bcmrpi3     : <unknown>
 - - debug       : Enable extra debug codepaths, like asserts and extra output. If you want to get meaningful backtraces see https://wiki.gentoo.org/wiki/Project:Quality_Assurance/Backtraces
 - - initramfs   : <unknown>
 - - savedconfig : Use this to restore your config from /etc/portage/savedconfig ${CATEGORY}/${PN}. Make sure your USE flags allow for appropriate dependencies
 - - test        : Enable dependencies and/or preparations necessary to run tests (usually controlled by FEATURES=test but can be toggled independently)

[samip5]

How do we even add the metadata for these?