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docs: Mention platform-specific predefined macros

Simon McVittie requested to merge wip/smcv/os-detect into main

The platform-specific predefined macros provided by various compilers sometimes capture subtle differences of meaning, like the distinction between the Linux kernel and a glibc-based (GNU/Linux) user-space. It would be difficult to capture those subtleties in GLib-specific convenience macros, particularly for platforms that we don't use ourselves.

Instead, recommend that anyone who is already writing platform-specific code should use the platform-specific predefined macros directly.

Alternative to !2986 (closed).

/cc @pwithnall @3v1n0 @matthiasc

This is basically just documenting the approach that @matthiasc advocated on !2986 (closed) as being official. I personally like this better than !2986 (closed).

I actually wonder whether we should also remove the new G_OS_DARWIN from the 2.75.x branch, and replace uses of it with __APPLE__, reverting most of !2981 (merged). Strictly speaking __APPLE__ could either be Mac OS Classic (ye olde cooperatively-multitasking, non-privilege-separated OS with its various non-Unix oddities) or the Darwin family (iOS, tvOS and modern macOS), but GLib no longer supports Mac OS Classic anyway, so anyone who is compiling GLib can assume that __APPLE__ means the Darwin family.

One thing from !2981 (merged) that seems correct to keep is that it switched non-GUI stuff from HAVE_COCOA to G_OS_DARWIN, and we would want to swap that to __APPLE__ instead of bringing back HAVE_COCOA, because Cocoa is a specific GUI framework rather than an OS - so looking at HAVE_COCOA was a bit like choosing whether to compile GNOME-specific or freedesktop-specific code by looking at HAVE_GTK.

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