In reductions, src and dest must be same at all PEs

[jdinan]

In OpenSHMEM 1.4, we required that the source and dest pointers correspond to the same symmetric object:

The same dest and source arrays, and the same pWrk and pSync work arrays, must be passed to all PEs in the active set.

However, we lost this text in OpenSHMEM 1.5. This PR adds it back.