man: improve wording and comma usage in systemd.journal-fields(7)

Improve wording under "Description" and "_KERNEL_DEVICE="
drop several entries from kbd-model-map whose kbd layouts do not exist

kbd-model-map was generated from system-config-keyboard's keyboard_models.py.
Several of the kbd layouts referred in that file do not exist and, so far as I
can tell, never did. I believe these entries existed simply to provide the xkb
configuration information for those layouts, and there never were matching kbd
entries; the kbd names were entirely notional, to satisfy the need for some
entry or other in that field.

For systemd, the only function of kbd-model-map is to 'match' kbd and xkb
configurations, so it does not make any sense to maintain entries for cases
where only one or the other exists in this context.
correct name of Tajik kbd layout in kbd-model-map