Clarify that PMIX_NODE_SIZE counts processes that have not (or will not) call pmix

[abouteiller]

Clarify that PMIX_NODE_SIZE counts processes that have not (or will not) call pmix.

• [ ] decide if a change-log item is needed.

[jjhursey]

PMIx ASC 3Q 2023 Voting results:

• Yes: 5
• No: 0
• Abstain: 1
• Result: Pass as Errata. Can be merged at the discretion of Co-Chairs/RMs.