DocBook: update EXPORT_SYMBOL entry to point at export.h

Previously we used to get EXPORT_SYMBOL and friends from
module.h but we moved away from that since module.h largely
includes the entire header space one way or another.

As most users just wanted the simple export related macros,
they were spun off into a separate new header -- export.h
Update the docs to reflect that.

Suggested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Acked-by: Rob Landley <rob@landley.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>