Help: Document that cmake_minimum_required should be called first

Many projects put their project() call first, but the actions taken
by this command can be influenced by policies and other information
set by a call to cmake_minimum_required.  Document in each of these
two commands that cmake_minimum_required should be called first.

Suggested-by: Alan W. Irwin

Help/command/cmake_minimum_required.rst

@@ -28,3 +28,10 @@ which enables compatibility features for CMake 2.4 and lower.
 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.
+
+.. note::
+  Call the ``cmake_minimum_required()`` command at the beginning of
+  the top-level ``CMakeLists.txt`` file even before calling the
+  :command:`project` command.  It is important to establish version
+  and policy settings before invoking other commands whose behavior
+  they may affect.  See also policy :policy:`CMP0000`.

Help/command/project.rst

@@ -55,3 +55,10 @@ literal, direct call to the :command:`project` command; loading one
 through the :command:`include` command is not sufficient.  If no such
 call exists CMake will implicitly add one to the top that enables the
 default languages (``C`` and ``CXX``).
+
+.. note::
+  Call the :command:`cmake_minimum_required` command at the beginning
+  of the top-level ``CMakeLists.txt`` file even before calling the
+  ``project()`` command.  It is important to establish version and
+  policy settings before invoking other commands whose behavior they
+  may affect.  See also policy :policy:`CMP0000`.