Also fix MPI_Ibarrier.3in to .so to the right MPI_Barrier.3 page. Thanks to Maximilian for bringing the issue to our attention.
Add MPI_Ibarrier.3in to reference MPI_Barrier.3, and update MPI_Barrier.3in to include bindings for MPI_Ibarrier. Slightly update the text to be inclusive of the non-blocking case. Fixes #242.