kernel-doc: set verbose mode via environment

Honor the environment variable "KBUILD_VERBOSE=1" (as set by make V=1) to
enable verbose mode in scripts/kernel-doc.  Useful for getting more info and
warnings from kernel-doc.

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