kernel-doc: improve "no structured comments found" error

When using '!Ffile function' in a docbook template, and the function no
longer exists, you get a "no structured comments found" error from the
kernel-doc processing script.  It's useful to know which functions it was
looking for, so print them out in this case.  Also do the same for '!Pfile
doc-section'

The same error also happens when using '!Efile' when some exported
functions aren't documented (in the same file.) There's a very large
number of such functions though, so don't print the message in this case
-- right now it would give ~850 messages.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Cc: Rob Landley <rob@landley.net>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>