Help: Clarify how language-specific flag environment values are used

Brad Kingrequested to merge
brad.king/cmake:doc-flag-init into master

Previous the wording could be interpreted to mean that the environment variables like CXXFLAGS are used exclusively to initialize the corresponding cache entries like CMAKE_CXX_FLAGS. State clearly that the value will be used in combination with builtin defaults.

Issue: #23956 (closed)
Backport: release

Edited by Brad King

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