Documented cache_variables in CMakeToolchain (#2611)

* Documented cache_variables in CMakeToolchain

* Update reference/conanfile/tools/cmake/cmaketoolchain.rst

Co-authored-by: James <james@conan.io>

* Update reference/conanfile/tools/cmake/cmaketoolchain.rst

Co-authored-by: Francisco Ramírez <franchuti688@gmail.com>

Co-authored-by: James <james@conan.io>
Co-authored-by: Carlos Zoido <mrgalleta@gmail.com>
Co-authored-by: Francisco Ramírez <franchuti688@gmail.com>

reference/conanfile/tools/cmake/cmaketoolchain.rst

@@ -150,6 +150,24 @@ The booleans assigned to a variable will be translated to ``ON`` and ``OFF`` sym
 Will generate the sentences: ``set(FOO ON ...)`` and ``set(VAR OFF ...)``.
 
 
+cache_variables
++++++++++++++++
+
+This attribute allows defining CMake cache-variables. These variables, unlike the ``variables``, are single-config. They
+will be stored in the ``CMakePresets.json`` file (at the `cacheVariables` in the `configurePreset`) and will be
+applied with ``-D`` arguments when calling ``cmake.configure`` using the :ref:`CMake() build helper<conan-cmake-build-helper>`.
+
+
+.. code:: python
+
+    def generate(self):
+        tc = CMakeToolchain(self)
+        tc.cache_variables["foo"] = True
+        tc.cache_variables["foo2"] = False
+        tc.cache_variables["var"] = "23"
+
+The booleans assigned to a cache_variable will be translated to ``ON`` and ``OFF`` symbols in CMake.
+
 
 Generators
 ++++++++++