Clarify about isolation.

Author: ericsnowcurrently

Doc/library/interpreters.rst

@@ -50,6 +50,10 @@ to keep in mind about using multiple interpreters:
 
 .. XXX Are there other relevant details to list?
 
+In the context of multiple interpreters, "isolated" means each
+interpreter shares no state between them.  In practice, there is some
+process-global data they all share, but that is managed by the runtime.
+
 
 Reference
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