explain MPI support for C++

[jeffhammond]

Problem

We removed the C++ bindings. MPI 3.0 and later makes no reference to C++ except the datatypes (bool and std::complex<x> for x=float,double,long double. Many people are using MPI in C++ codes, using the C bindings. Similar to #139, we should specify the language reference for C++.

Proposal

Describe in words how MPI supports C++ and what we mean by C++. Cite the appropriate ISO C++ document.

Explain how MPI C bindings are used in C++ and how one should reason about support for post-C89 features like fixed-width integers.

Changes to the Text

TODO

Impact on Implementations

This should help Microsoft by explaining that they have all the necessary C99 language features referenced by MPI via C++11.

Impact on Users

It will be easier to reason about MPI-compiler interactions in C++ applications.

References

#139

[jeffhammond]

See #139...

[omor1]

@jeffhammond I think you're referring to the comment I made and promptly deleted once I read #139 a minute later. In other words, I should actually fully read the issue before commenting on it. Especially since it was my own prior comment that inspired both #139 and #140.