documentation of _shipped and offset header files

documentation of _shipped and offset header files

This proposed patch set describes the intent and use of _shipped files and offset header files in the kernel build system. The three parts are:

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

thx!
hofrat