pep-567: Add another reason why Contexts are not mutable (#623)

pep-0567.rst

@@ -712,6 +712,15 @@ This proposal was deferred to Python 3.8+ because of the following:
 
      ctx.run(func)
 
+3. If ``Context`` was mutable it would mean that context variables
+   could be mutated separately (or concurrently) from the code that
+   runs within the context.  That would be similar to obtaining a
+   reference to a running Python frame object and modifying its
+   ``f_locals`` from another OS thread.  Having one single way to
+   assign values to context variables makes contexts conceptually
+   simpler and more predictable, while keeping the door open for
+   future performance optimizations.
+
 
 Having initial values for ContextVars
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