Missing primary index and description of MPI_T_source_order

[wgropp]

Problem

In reviewing the MPI 4.1 RC, I noticed that MPI_T_source_order doesn't have a primary index entry. In reviewing the document, this is because there is no discussion of this type, just a regular index entry macro.

Proposal

Follow the approach used for other MPI_T types and ensure that there is an mpictypemain entry for MPI_T_source_order.

Changes to the Text

For the CC to consider.

Impact on Implementations

None.

Impact on Users

Improved description of the standard

References and Pull Requests