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.
Add documentation for both the CMake variable and environment variable
of this name pattern. Update references to these names to link to their
documents. Clarify the pattern used to construct their names.
Nikita Nemkin