document how to pass MPI.Comm objects to C (#626)

Co-authored-by: Simon Byrne <simonbyrne@gmail.com>

Author: stevengj

docs/src/reference/comm.md

@@ -12,6 +12,8 @@ each process a unique *rank* (see [`MPI.Comm_rank`](@ref)) taking an integer val
 MPI.Comm
 ```
 
+If you need to pass a Julia `MPI.Comm` object to an external C/C++ library ([via `ccall`](https://docs.julialang.org/en/v1/manual/calling-c-and-fortran-code/)) that expects an `MPI_Comm` argument, you should declare the corresponding `ccall` argument type `MPI.API.MPI_Comm` and the correct conversion will be performed.
+
 ## Constants
 
 ```@docs