Docs for cmake_extra_variables property (#4257)

* Docs for cmake_extra_variables property

* Code example

Author: AbrilRBS

reference/tools/cmake/cmakedeps.rst

@@ -252,6 +252,12 @@ The following properties affect the CMakeDeps generator:
 - **cmake_additional_variables_prefixes**: List of prefixes to be used when creating CMake variables in the config
   files. These variables are created with ``file_name`` as prefix by default, but setting this property will create
   additional variables with the specified prefixes alongside the default ``file_name`` one.
+- **cmake_extra_variables**: Dictionary of extra variables to be added to the generated config file.
+  The keys of the dictionary are the variable names and the values are the variable values,
+  which can be a plain string, a number or a dict-like python object which must specify
+  the ``value`` (string/number) , ``cache`` (boolean), ``type`` (CMake cache type) and optionally,
+  ``docstring`` (string: defaulted to variable name) and ``force`` (boolean) keys. Note that this has
+  less preference over those values defined in the ``tools.cmake.cmaketoolchain:extra_variables`` conf.
 
 Example:
 
@@ -282,6 +288,14 @@ Example:
         # Generate both MyFileNameConfig.cmake and FindMyFileName.cmake
         self.cpp_info.set_property("cmake_find_mode", "both")
 
+        # Add extra variables to the generated config file
+        self.cpp_info.set_property("cmake_extra_variables", {
+                                       "FOO": 42,
+                                       "CMAKE_GENERATOR_INSTANCE": "${GENERATOR_INSTANCE}/buildTools/",
+                                       "CACHE_VAR_DEFAULT_DOC": {"value": "hello world",
+                                                                 "cache": True, "type": "STRING"}
+                                   })
+
 
 Overwrite properties from the consumer side using CMakeDeps.set_property()
 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++