Improve documentation of the type safety attributes.

Improve documentation of the type safety attributes.
1. Add description of the arguments to these attributes.
2. Add missing declarations to some of the MPI code examples.
3. Made clarifications where possible.

Based on the write-up by: Craig Flores.

Differential Revision: https://reviews.llvm.org/D22717

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@277192 91177308-0d34-0410-b5e6-96231b3b80d8