Adding note on SB undefined behavior with SB_PEND_FOREVER and CFE_SB_DeletePipe()

[jbohren-hbr]

Describe the contribution
This adds affirmative documentation regarding the discussion in #1777 and #1799

Testing performed
Documentation only.

Expected behavior changes
Documentation only.

System(s) tested on

• None

Additional context
See discussion in #1777

Third party code
No third party code included.

Contributor Info - All information REQUIRED for consideration of pull request
Jonathan Bohren, Honeybee Robotics

[astrogeco]

CCB:2021-08-18

• Postpone until Draco
• App developers shouldn't call "pend forever" and ensure a graceful shutdown
• It is hard to handle this in a generic sense
• Try to avoid managing resources from multiple places
• Make a note on all Delete APIs of what happens when calling them when a resource is shared
• Might be able to handle it by sending a message to all subscribers -> this will probably break things, devil is in the details...
• Unix signals "got it right"

[skliper]

Short story - although these statements are true (and kudos for using a functional test to show the behavior!!), there's hesitation in quickly merging such information when we'd prefer to take the time to document/detail supported patterns and things to avoid when sharing resources across tasks from a more holistic perspective. Definitely appreciate the contribution and we plan to circle back.

[jbohren-hbr]

Sounds good. Thanks for the updates @skliper @astrogeco.