RMA: Examples 12.4 and 12.5 contain advice to implementors

[devreal]

Problem

@RolfRabenseifner brought up this issue.

The Examples 12.4 and 12.5 essentially contain text that give advice to implementors on which functions may delay their return. The text should be taken out of the example and into an AtoI, with an AtoU clarifying the important aspects for users.

Proposal

Move most of the example text into AtoI and add an AtoU.

Changes to the Text

See above.

Impact on Implementations

No impact to implementations.

Impact on Users

Easier to grasp the important aspects of the examples.

References and Pull Requests

https://github.com/mpi-forum/mpi-standard/pull/768