Clarify about running interpreters. (#402)

pep-0554.rst

@@ -132,16 +132,20 @@ The module also provides the following classes:
 
    is_running():
 
-      Return whether or not the interpreter is currently running.
+      Return whether or not the interpreter is currently executing code.
 
    destroy():
 
-      Finalize and destroy the interpreter.
+      Finalize and destroy the interpreter.  If called on a running
+      interpreter, RuntimeError will be raised.
 
    run(code):
 
       Run the provided Python code in the interpreter, in the current
-      OS thread.  Supported code: source text.
+      OS thread.  If the interpreter is already running then raise
+      RuntimeError.
+
+      Supported code: source text.
 
    get_fifo(name):