Removal of const in signal set, add and minor changes

[wrrobin]

Summary of changes

This PR address issue #509. Also, restructures signaling section. Adds a minor change to the sig_addr description of shmem_signal_wait_until.

Proposal Checklist

• [ ] Link to issue(s)
• [ ] Changelog entry
• [ ] Reviewed for changes to front matter
• [ ] Reviewed for changes to back matter

[jdinan]

@naveen-rn Please review to determine if these changes can be handled by the signals section committee.

[wrrobin]

Closing this as the changes are merged in https://github.com/naveen-rn/specification/tree/osm1.6/rc1-rma-amo-pws and will be opened as PR once ready as section edits.