kernel-doc: detect trailing kernel-doc line trash

Print a warning when a kernel-doc comment block ends with text on the same
line as the ending comment characters, e.g.:

 * this text is lost. */

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>