rfc13: need to sync up with MPICH PMI changes and documentation

[garlick]

We need to track any changes to the mpich wire protocol. This new document in the mpich source describes a PMI 1.1 wire protocol that differs quite a bit from what we have now. However we have only found one interoperability problem between that version of mpich and flux so far.

https://github.com/pmodels/mpich/blob/main/src/pmi/maint/pmi-1.1.txt

See also pmodels/mpich#7050

[garlick]

See also:

• https://github.com/pmodels/mpich/blob/main/src/pmi/maint/pmi-1.2.txt
• implement PMI_PORT and support localhost:PORT and UNIX domain socket path flux-core#7184