docs: Do not include kerneldoc comments from kernel/sys.c

docs: Do not include kerneldoc comments from kernel/sys.c

...because there are none there, and I cannot figure out what would ever
have been of interest there.  This eliminates this warning:

    ./kernel/sys.c:1: warning: no structured comments found

from the build.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>