Document kernel taint flags properly

This fills in the documentation for all of the current kernel taint
flags, and fixes the number for TAINT_CRAP, which was incorrectly
described.

Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>