kernel-doc: preferred ending marker and examples

Fix kernel-doc-nano-HOWTO.txt to use */ as the ending marker in kernel-doc
examples and state that */ is the preferred ending marker.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Reported-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>