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>

Documentation/oops-tracing.txt

@@ -257,6 +257,8 @@ characters, each representing a particular tainted value.
 
  10: 'W' if a warning has previously been issued by the kernel.
 
+ 11: 'C' if a staging driver has been loaded.
+
 The primary reason for the 'Tainted: ' string is to tell kernel
 debuggers if this is a clean kernel or if anything unusual has
 occurred.  Tainting is permanent: even if an offending module is