Help: Add advice for dealing with semicolons in lists

Brad Kingrequested to merge
brad.king/cmake:doc-list-semicolons into master

Provide an inline example, and reference the ExternalProject LIST_SEPARATOR option as another example.

Also update the LIST_SEPARATOR documentation. Since CMAKE_ARGS is used to construct the default CONFIGURE_COMMAND for CMake-based external projects, the LIST_SEPARATOR option works for it too.

Issue: #23315
Backport: release

Edited by Brad King

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