add example showing impact of progress

[jeffhammond]

Problem

Following up on #634 and #635, we need an example that shows a program whose behavior depends on the attribute.

Proposal

Example like this:

One process does an RMA Put.
Another process spins on memory with Win_sync.

The completion of this program depends on strong progress.

Changes to the Text

TODO

Impact on Implementations

None

Impact on Users

Makes the world make sense.

References and Pull Requests

TODO

[Wee-Free-Scot]

Depends on #635 -- the proposed example demonstrates the effect of different settings for the attribute proposed in #635.

Before #635, this example would have to say "always an unsafe program because MPI progress may not happen".

Then #635 would modify that brand new text to "example program is unsafe only when attribute is set to 'weak'; safe otherwise".