docs: cec osd name pitfalls and the ostree video group trap

[Weijtmans]

Two follow-ups to the CEC section from #45, both hit on the same setup and both cost real time to figure out.

The TV shows the box as "Playback", and the obvious fix does not work. On the kernel-CEC path a device gets cec-ctl's default OSD name unless --osd-name is passed. On Bazzite the documented-looking knob, CEC_OSD_NAME in /etc/default/cec-control, is a no-op: reading /usr/bin/cec-control shows it builds --osd-name into an argument list that only the libcec code path uses, while the /dev/cec0 path (the one the BC-250 takes) runs a bare cec-ctl --playback. On top of that, cec-onboot.service re-runs the bare registration every boot, clobbering a manually set name. The new subsection gives the direct cec-ctl command, the 14-character limit, and links the small tool that persists the name past cec-onboot. It also sets expectations: renaming does not change the device category, CEC has no game-console type and amdgpu has no ALLM, so Game Mode is a TV-side input label.

The "add your user to the video group" step silently fails on ostree distros. With nss-altfiles the video group often lives only in the read-only /usr/lib/group; usermod -aG validates the group through NSS, then edits /etc/group, finds no line, and exits 0 having changed nothing. The warning gives the two-line fix (seed the line, then gpasswd -a).

Both are first-party: the cec-control behaviour is from reading the shipped script, and the usermod trap was hit and fixed on the tested board.

mkdocs build --strict passes.

Tested by: @Weijtmans. BC-250, Bazzite (Fedora Atomic 43), kernel 6.17.7-ba29, Samsung TV.