kernel-doc: fix leading dot in man-mode output

If a parameter description begins with a '.', this indicates a "request"
for "man" mode output (*roff), so it needs special handling.

Problem case is in include/asm-i386/atomic.h for function
atomic_add_unless():
 * @u: ...unless v is equal to u.
This parameter description is currently not printed in man mode output.

[akpm@linux-foundation.org: cleanup]
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>