Fix documentation & filter for MVmap "get" and "iterator"
The documentation of MVmap is inadequate
- Explain when/why MVmap get or iterator returns a set of values (concurrent writes) or a single value (last assign overwrote previous writes).
- Explain when one of those values is a null (concurrent delete)
- document that "%" in keys is special and must be avoided. Add an assert to that effect.
- Clarify that get or iterator does not return an MVregister; explain why; provide a convert/cast interface.
Edited by Marc Shapiro