PEP-654: state in rationale the premise that the new features will be…

… used selectively (#1851)

pep-0654.rst

@@ -119,6 +119,13 @@ The purpose of this PEP, then, is to add the ``except*`` syntax for handling
 handling semantics, so we are not proposing to modify the behavior of the
 ``except`` keyword but rather to add the new ``except*`` syntax.
 
+Our premise is that ``ExceptionGroups`` and ``except*`` will be used
+selectively, only when they are needed. We do not expect them to become
+the default mechanism for exception handling.  The decision to raise
+``ExceptionGroup`` from a library needs to be considered carefully and
+regarded as an API-breaking change. We expect that this will normally be
+done by introducing a new API rather than modifying an existing one.
+
 
 Specification
 =============