Clarify that the API documentation is the public API.

Author: kotfu

docs/api/index.rst

@@ -1,6 +1,19 @@
 API Reference
 =============
 
+These pages document the public API for ``cmd2``. If a method, class, function,
+attribute, or constant is not documented here, consider it private and subject
+to change. There are many classes, methods, functions, and constants in the
+source code which do not begin with an underscore but are not documented here.
+When looking at the source code for this library, you can not safely assume
+that because something doesn't start with an underscore, it is a public API.
+
+If a release of this library changes any of the items documented here, the
+version number will be incremented according to the `Semantic Version
+Specification <https://semver.org>`_.
+
+This documentation is for version |version| of ``cmd2``.
+
 .. toctree::
    :maxdepth: 1
 

docs/overview/integrating.rst

@@ -10,8 +10,11 @@ include ``cmd2``. Make sure your ``setup.py`` includes the following::
 
 The ``cmd2`` project uses `Semantic Versioning <https://semver.org>`_, which
 means that any incompatible API changes will be release with a new major
-version number. We recommend that you follow the advice given by the Python
-Packaging User Guide related to `install_requires
+version number. The public API is documented in the :ref:`api/index:API
+Reference`.
+
+We recommend that you follow the advice given by the Python Packaging User
+Guide related to `install_requires
 <https://packaging.python.org/discussions/install-requires-vs-requirements/>`_.
 By setting an upper bound on the allowed version, you can ensure that your
 project does not inadvertently get installed with an incompatible future