docs/glossary: Elaborate on possible MicroPython port differences.

State that this doc describes generic, "core" MicroPython functionality,
any particular port may diverge in both directions, by both omitting
some functionality, and adding more, both cases described outside the
generic documentation.

Author: pfalcon

docs/reference/glossary.rst

@@ -68,7 +68,13 @@ Glossary
         MicroPython supports different :term:`boards <board>`, RTOSes,
         and OSes, and can be relatively easily adapted to new systems.
         MicroPython with support for a particular system is called a
-        "port" to that system.
+        "port" to that system. Different ports may have widely different
+        functionality. This documentation is intended to be a reference
+        of the generic APIs available across different ports ("MicroPython
+        core"). Note that some ports may still omit some APIs described
+        here (e.g. due to resource constraints). Any such differences,
+        and port-specific extensions beyond MicroPython core functionality,
+        would be described in the separate port-specific documentation.
 
     MicroPython Unix port
         Unix port is one of the major :term:`MicroPython ports <MicroPython port>`.