Fixed ticket 2723

[SVN r55445]
proposed fix for ticket 2399

[SVN r55449]
Make MPI non-blocking semantics more clear

[SVN r56203]

Conflicts:
	doc/mpi.qbk
Merge pull request #4 from danieljames/master-cherry-pick

Master cherry pick

Looks good Daniel, thanks for putting this together.  I don't see any problem with applying these changes.