Clarify that calling MPI_INFO_SET with MPI_INFO_ENV is erroneous

[raffenet]

Problem

MPI_INFO_ENV is a predefined MPI object. It cannot be freed by the user. Allowing the user to update the key/value pairs in it is unintuitive, but both MPICH and Open MPI allow it.

Proposal

Clarify that calling MPI_INFO_SET with MPI_INFO_ENV is not allowed.

Changes to the Text

TBD. Should be minor.

Impact on Implementations

Add an extra error check.

Impact on Users

None?

References and Pull Requests

TBD