Update Group Construct entry

[Ralph Castain]

Hello all

I have filed a change that adds a considerable amount of explanation to the new group “bootstrap” construct method. The PR also updates the PMIx_Group_construct entry to include a few missing attributes (provided as provisional):

* PMIX_GROUP_LOCAL_CID - allow a participant to provide a local context ID to be shared with other participants

* PMIX_GROUP_BOOTSTRAP - indicate that the "bootstrap" method of group construction is to be used

* PMIX_GROUP_ADD_MEMBERS - specify the additional members that are to be included in the final group

* PMIX_GROUP_JOB_INFO - used in the data returned at the end of the group construct operation. Includes all job-level info for participants.

The PR is available here: https://github.com/pmix/pmix-standard/pull/520

Please feel free to comment and/or vote your preference

Ralph