documentation of _shipped and offset header files

This patchset cleanup includes documentation updates for _shipped files, offset header files as well as some directly related change proposals in Kbuild and CodingStyle.

This proposed patch set describes the intent and use of _shipped files and offset header files as well as a minor update on the handling of external module dependencies.

- an additional clause to Documentation/CodingStyle for _shipped files
- a description of _shipped in Documentation/kbuild/makefiles.txt
- documentation of offset header files
- change to scripts/Makefile.lib to restrict _shipped to .c .h and .S files and a special (historic) .incl_shipped case
- fix wording in Documentation/kbuild/modules.txt on _shipped files

thx!
hofrat