Update Group Construct entry

Update the PMIx_Group_construct entry to include a few missing
attributes:

* 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.

Extend the PMIx_Group_construct description to explain the new
construct method and how it is used.

Signed-off-by: Ralph Castain <[email protected]>

Author: rhc54