docs: Describe the 'C' taint flag in oops-tracing.txt

If some of the flags are documented there, they all should be.

Signed-off-by: Nick Bowler <nbowler@elliptictech.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>