Tweak the text on API Docs intro page [skip appveyor]

Signed-off-by: Mats Wichmann <mats@linux.com>

Author: mwichmann

doc/sphinx/index.rst

@@ -7,21 +7,37 @@ SCons API Documentation
 =======================
 
 .. Attention::
-   This is the **internal** API Documentation for SCons.
-   The documentation is automatically generated for each release
-   from the source code using the
-   `Sphinx <https://www.sphinx-doc.org>`_ documentation generator.
-   Missing information is due to shortcomings in the docstrings in the code,
-   which admittedly could use a lot more work (contributions welcomed!).
-
-   The target audience is both developers working on SCons itself,
-   and those writing external Tools, Builders, etc. and other
-   related functionality, who need to reach beyond the Public API.
-   Note that what is Public API is not clearly deliniated in the API Docs.
-   The interfaces available for use in SCons configuration scripts
-   ("SConscript files"), which have a consistency guarantee,
-   are those documented in the `SCons Reference Manual
-   <https://scons.org/doc/production/HTML/scons-man.html>`_.
+   This is the **internal** API Documentation for SCons (aka
+   "everything"). It is generated automatically from code docstrings using
+   the `Sphinx <https://www.sphinx-doc.org>`_ documentation generator.
+
+   Any missing/incomplete information is due to shortcomings in the
+   docstrings in the code. To not be too flippant about it, filling
+   in all the docstrings has not always been a priority across the
+   two-plus decades SCons has been in existence (contributions on this
+   front are welcomed).  Additionally, for SCons classes which inherit
+   from Python standard library classes (such as ``UserList``,
+   ``UserDict``, ``UserString``), the generated pages will show methods
+   that are inherited, sometimes with no information at all, sometimes
+   with a signature/description that seems mangled: Python upstream has
+   similar limitations as to the quality of dosctrings vs the current
+   standards Sphinx expects.  Inherited interfaces can be identified by the
+   lack of a ```[source]``` button to the right of the method signature.
+
+   If you are looking for the Public API - the interfaces that have
+   long-term consistency guarantees, which you can reliably use when
+   writing a build system for a project - see the `SCons Reference Manual
+   <https://scons.org/doc/production/HTML/scons-man.html>`_.  Note that
+   what is Public API and what is not is not clearly delineated in these
+   API Docs.
+
+   The target audience is both developers contributing to SCons itself,
+   and those writing external Tools, Builders, and other related
+   functionality for their project, who may need to reach beyond the
+   Public API to accomplish their tasks. Reaching into internals is fine,
+   but comes with the usual risks of "things here could change, it's up
+   to you to keep your code working".
+
 
 .. toctree::
    :maxdepth: 2