Help: Document special cases for if(IS_ABSOLUTE)

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.
33 jobs for !6016 with doc-if-IS_ABSOLUTE in 6 minutes and 6 seconds (queued for 2 seconds)
latest detached
Status Name Job ID Coverage
  Build
manual build:centos6-x86_64 #5712412
cmake build docker linux manual
passed build:debian10-iwyu #5712409
cmake build docker linux

00:01:33

manual build:fedora33-ninja #5712413
cmake build docker linux linux-3.17 manual
passed build:fedora33-sphinx #5712411
docker linux linux-3.17 cmake build

00:03:25

passed build:fedora33-tidy #5712410
cmake build docker linux linux-3.17

00:06:06

manual build:macos-x86_64-makefiles #5712415
cmake macos shell xcode-12.4 nonconcurrent manual
manual build:macos-x86_64-ninja #5712414
cmake macos shell xcode-12.4 nonconcurrent manual
manual build:windows-vs2019-x64-ninja #5712416
cmake windows shell vs2019 msvc-19.28-16.9 nonconcurrent manual
 
  Test
created test:fedora33-ninja #5712417
cmake docker linux linux-3.17 x11
created test:macos-x86_64-makefiles #5712419
cmake macos shell xcode-12.4 nonconcurrent
created test:macos-x86_64-ninja #5712418
cmake macos shell xcode-12.4 nonconcurrent
created test:windows-vs2019-x64-ninja #5712420
cmake windows shell vs2019 msvc-19.28-16.9 nonconcurrent
 
  Test Ext
created test:cuda10.2-nvidia #5712423
cmake cuda-rt docker linux
created test:debian10-ninja #5712421
cmake build docker linux linux-3.17
created test:fedora33-makefiles #5712422
cmake build docker linux linux-3.17
created test:fedora33-ninja-multi #5712424
cmake build docker linux linux-3.17
created test:macos-x86_64-xcode #5712425
cmake macos shell xcode-12.4 concurrent
created test:windows-vs2019-x64 #5712426
cmake windows shell vs2019 msvc-19.28-16.9 concurrent
 
  External
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