pep-567: More fixes (#505)

pep-0567.rst

@@ -78,16 +78,15 @@ is accessed via ``contextvars.get_context()`` function.
 Manipulation of the current ``Context`` is the responsibility of the
 task framework, e.g. asyncio.
 
-A ``Context`` is conceptually a mapping, implemented using an
-immutable dictionary.  The ``ContextVar.get()`` method does a
+A ``Context`` is conceptually a read-only mapping, implemented using
+an immutable dictionary.  The ``ContextVar.get()`` method does a
 lookup in the current ``Context`` with ``self`` as a key, raising a
 ``LookupError``  or returning a default value specified in
 the constructor.
 
 The ``ContextVar.set(value)`` method clones the current ``Context``,
 assigns the ``value`` to it with ``self`` as a key, and sets the
-new ``Context`` as a new current.  Because ``Context`` uses an
-immutable dictionary, cloning it is O(1).
+new ``Context`` as a new current.
 
 
 Specification
@@ -221,9 +220,9 @@ keyword-only argument, which defaults to the current context::
         # ... some time later
         context.run(callback, *args)
 
-Tasks in asyncio need to maintain their own isolated context that
-they inherit from the point they were created at.  ``asyncio.Task``
-is modified as follows::
+Tasks in asyncio need to maintain their own context that they inherit
+from the point they were created at.  ``asyncio.Task`` is modified
+as follows::
 
     class Task:
         def __init__(self, coro):
@@ -395,7 +394,7 @@ Summary of the New APIs
   parameter can be used to specify a custom context.
 
 * ``asyncio.Task`` is modified internally to maintain its own
-  isolated context.
+  context.
 
 
 Backwards Compatibility