kernel-doc: suppress 'not described' warnings for embedded struct fields

The ability to add kerneldoc comments for fields in embedded structures is
useful, but it brought along a whole bunch of warnings for fields that
could not be described before.  In many cases, there's little value in
adding docs for these nested fields, and in cases like:

       	struct a {
            struct b {
	        int c;
	    } d, e;
	};

"c" would have to be described twice (as d.c and e.c) to make the warnings
go away.

We can no doubt do something smarter, but simply suppressing the warnings
for this case removes about 70 warnings from the docs build, freeing us to
focus on the ones that matter more.  So make kerneldoc be silent about
missing descriptions for any field containing a ".".

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