Doc: specify what happens for Z/M for AddPoint() / AddPoint_2D() #7569

Job	Run time
build-linux-ubuntu-focal	1h 6m 24s
build-windows-conda	28m 23s
build-windows-minimum	37m 51s
build-windows-msys2-mingw	34m 20s
build-mac	12m 52s
build-windows-conda-csharp-only	0s
	2h 59m 50s

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