Help: xref variable that cmake_minimum_required() sets

The CMAKE_MINIMUM_REQUIRED_VERSION has long been set by
the cmake_minimum_required() command, but the command docs
never mentioned this. Only the variable's own docs stated this but
the variable isn't very discoverable without it being mentioned in
the command's docs as well.

Help/command/cmake_minimum_required.rst

@@ -22,6 +22,9 @@ dots will be seen as version component separators, resulting in the
 ``...<max>`` part being ignored and preserving the pre-3.12 behavior
 of basing policies on ``<min>``.
 
+This command will set the value of the
+:variable:`CMAKE_MINIMUM_REQUIRED_VERSION` variable to ``<min>``.
+
 The ``FATAL_ERROR`` option is accepted but ignored by CMake 2.6 and
 higher.  It should be specified so CMake versions 2.4 and lower fail
 with an error instead of just a warning.