checkpatch: clarify the EMBEDDED_FUNCTION_NAME message

Try to make the conversion of embedded function names to "%s: ", __func__
a bit clearer.

Add a bit more information to the comment describing the test too.

Link: http://lkml.kernel.org/r/38f5d32f0aec1cd98cb9ceeedd6a736cc9a802db.1491759835.git.joe@perches.com
Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>