Help: Document special cases for if(IS_ABSOLUTE)

Craig Scott requested to merge craig.scott/cmake:doc-if-IS_ABSOLUTE into master

The meaning of "absolute path" was previously assumed knowledge, but a number of special cases were left unspecified. The way some of these are handled differs to the way that cmake_path(IS_ABSOLUTE) works, so document those special cases so that the differing behavior between these two commands is clearly defined. Since cmake_path() was added in CMake 3.20 and this is about clarifying things that are closely related to the new command, I've set this MR to backport to the release branch.

There's a small drive-by fix for file(TO_NATIVE_PATH) as well, found while checking for other similar platform-specific behavior of commands that overlap with cmake_path() functionality.

Backport: release

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