Revert "Help: Document possible values of CMAKE_<LANG>_PLATFORM_ID (#15012)"

This reverts commit d2f68588caba8f9df5756fc5c52d6e54d2caa6a5.
This variable is still supposed to be internal.  CMAKE_SYSTEM_NAME
is the public one.
Merge topic 'doc-CMAKE_LANG_PLATFORM_ID' into next

cb6551ea Revert "Help: Document possible values of CMAKE_<LANG>_PLATFORM_ID (#15012)"
Help: Fix PLATFORM_ID corresponding var
Merge topic 'doc-genex-id-values' into next

6444f8be Help: Fix PLATFORM_ID corresponding var
Help: Link to variables corresponding to id and version expressions (#15012)

In the cmake-generator-expressions(7) manual, link to the variables
that correspond to the PLATFORM_ID, C_COMPILER_ID, CXX_COMPILER_ID,
C_COMPILER_VERSION, and CXX_COMPILER_VERSION generator expressions.
Merge topic 'doc-genex-id-values' into next

5690a963 Help: Link to variables corresponding to id and version expressions (#15012)