Commit ab0e9dbfa7 introduced some changes
to the documentation Makefiles designed to clean-up the process of
deciding which headers get scanned for the docs.
Unfortunately, the gtk-doc Makefile doesn't use HFILE_GLOB for actually
generating the docs -- only for knowing when it needs to redo the
generation. Because of this, we need to use IGNORE_HFILES or otherwise
we get hundreds of symbols in the *-unused.txt files.
Revert the changes that that commit made to the docs Makefiles (but
leave the generation of the *-public-headers.txt files in place).
b407086b38
