Add some documentation on Mapepire

[SJLennon]

Since the latest versions rely heavily on Mapepire, and some issues and misunderstanding of server vs single use have occurred, I think it would be good to add some documentation about Mapepire.

(Incidentally, if you search for Mapepire in the documentation it gives several hits, but none seem to be even slightly relevant. If I search the source Mapepire appears twice, but both are in a hyperlink in the DB2 section.)