[sv-ac] diffs for mantis 1668

All:

I forgot to mention what I changed for those who may
already have looked at the old 1668 proposal (which I
also deleted).

1. Added an example illustrating the use of an inc_or_dec_expression
   to update a local variable.

2. Added clarifying discussion of when a local variable can be
   referenced.
   - it must already have been assigned
   - the use of an inc_or_dec_expression or an assignment operator
     other than "=" involves an implicit reference

3. Added language correlating the concepts of "not flowing" with
   "becoming unassigned".

4. Added comments to help clarify which of a pair of examples is the
   legal code and which is the illegal code.

J.H.

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