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
Many environment variables were documented late and got
assigned wrong versions by the script.
(The whole Help/envvar section was only added in 3.10).
Issue: #19715
In each environment variable doc page, insert an opening
paragraph (ENV_VAR.txt) to say that this is an environment
variable, and provide a link to the cmake-language section
on environment variables.
Ilia K