doc: clarify the wording on <git-compat-util.h> requirement

The reason why we require the <git-compat-util.h> file to be the first header file to be included is because it insulates other header files and source files from platform differences, like which system header files must be included in what order, and what C preprocessor feature macros must be defined to trigger certain features we want out of the system. We tried to clarify the rule in the coding guidelines document, but the wording was a bit fuzzy that can lead to misinterpretations like you can include <xdiff/xinclude.h> only to avoid having to include <git-compat-util.h> even if you have nothing to do with the xdiff implementation, for example. "You do not have to include more than one of these" was also misleading and would have been puzzling if you _needed_ to depend on more than one of these approved headers (answer: you are allowed to include them all if you need the declarations in them for reasons other than that you want to avoid including compat-util yourself). Instead of using the phrase "approved headers", enumerate them as exceptions, each labeled with its intended audiences, to avoid such misinterpretations. The structure also makes it easier to add new exceptions, so add the description of "t/unit-tests/test-lib.h" being an exception only for the unit tests implementation as an example. Signed-off-by: Junio C Hamano <gitster@pobox.com> Acked-by: Kyle Lippincott <spectral@google.com> Acked-by: Elijah Newren <newren@gmail.com>