docs: more words about the public api

Author: nedbat

doc/api.rst

@@ -39,9 +39,15 @@ files.
 
 .. note::
 
-    Only the documented portions of the API are supported. Other names you may
-    find in modules or objects can change their behavior at any time. Please
-    limit yourself to documented methods to avoid problems.
+    Only the published documented portions of the API are supported. Other
+    names you may find in modules or objects can change their behavior at any
+    time. Please limit yourself to documented methods to avoid problems.
+
+    All internal code in coverage.py has docstrings; this does not make them
+    part of the public supported API.  Many internal names have no leading
+    underscore; this does not make them part of the public supported API.  If
+    classes or functions are not documented in the published documentation,
+    they are not supported.
 
 For more intensive data use, you might want to access the coverage.py database
 file directly.  The schema is subject to change, so this is for advanced uses