Documentation | Clarify that message broadcast is non-atomic (!196)

[Martin Werner (@martinerikwerner)]

Martin Werner created a merge request: https://gitlab.rtems.org/rtems/docs/rtems-docs/-/merge_requests/196

Project:Branches: martinerikwerner/rtems-docs:msg-broadcast-is-non-atomic to rtems/docs/rtems-docs:main
Author:   Martin Werner




## Summary

**c-user/message/operations.md: Don't claim atomic**

rtems_message_queue_broadcast() is not atomic, since it unblocks each
task after copying the message into their message buffer. So remove the
wording indicating that it is atomic.

Also reword the overall description slightly with "each" instead of
"every", and add "until no more tasks remain", in order to further hint
at the non-atomicity of the operation.

(See https://gitlab.rtems.org/rtems/rtos/rtems/-/issues/4804)

**c-user/message/directives.md: Note non-atomicity**

rtems_message_queue_broadcast() may, under certain circumstances, copy
the message to tasks which were not waiting on the message queue when
the broadcast started, and may copy the message multiple times to the
same task.

This behaviour is, based on discussion in
https://gitlab.rtems.org/rtems/rtos/rtems/-/issues/4804, something that
might not change in the short term, so expose it in the user
documentation.

The wording is copied with modifications from an existing internal
requirements note.

The changes were auto-generated from rtems-central.

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