Help: Describe $CACHE and $ENV as operators

This resolves issue #18514

Also add a cross-reference to if(DEFINED ENV{var}).

Help/manual/cmake-variables.7.rst

@@ -241,7 +241,6 @@ Variables that Describe the System
    /variable/ANDROID
    /variable/APPLE
    /variable/BORLAND
-   /variable/CACHE
    /variable/CMAKE_CL_64
    /variable/CMAKE_COMPILER_2005
    /variable/CMAKE_HOST_APPLE
@@ -260,7 +259,6 @@ Variables that Describe the System
    /variable/CMAKE_SYSTEM_PROCESSOR
    /variable/CMAKE_SYSTEM_VERSION
    /variable/CYGWIN
-   /variable/ENV
    /variable/GHS-MULTI
    /variable/MINGW
    /variable/MSVC
@@ -599,3 +597,12 @@ Variables for CPack
    /variable/CPACK_PACKAGING_INSTALL_PREFIX
    /variable/CPACK_SET_DESTDIR
    /variable/CPACK_WARN_ON_ABSOLUTE_INSTALL_DESTINATION
+
+Variable Expansion Operators
+============================
+
+.. toctree::
+   :maxdepth: 1
+
+   /variable/CACHE
+   /variable/ENV

Help/variable/CACHE.rst

@@ -1,7 +1,7 @@
 CACHE
 -----
 
-Read cache variables.
+Operator to read cache variables.
 
 Use the syntax ``$CACHE{VAR}`` to read cache entry ``VAR``.
 See the :ref:`cmake-language(7) variables <CMake Language Variables>`
@@ -14,4 +14,5 @@ found CMake will search for a cache entry with that name.
 The ``$CACHE{VAR}`` syntax can be used to do direct cache lookup and
 ignore any existing normal variable.
 
-See the :command:`set` command to see how to write cache variables.
+See the :command:`set` and :command:`unset` commands to see how to
+write or remove cache variables.

Help/variable/ENV.rst

@@ -1,8 +1,12 @@
 ENV
 ---
 
-Read environment variables.
+Operator to read environment variables.
 
 Use the syntax ``$ENV{VAR}`` to read environment variable ``VAR``.
 
-See the :command:`set` command to see how to write environment variables.
+To test whether an environment variable is defined, use the signature
+``if(DEFINED ENV{<name>})`` of the :command:`if` command.
+
+See the :command:`set` and :command:`unset` commands to see how to
+write or remove environment variables.