Document the fact that ve.dm.MetaItem#replaceWith takes a plain object

Also encourage callers to pass plain objects unless they know what
they're doing; it's almost always wrong to pass in a MetaItem.

Change-Id: I8e8ef8ac7f77ec9f929d797e467b9c9d1140d721
Merge "Document the fact that ve.dm.MetaItem#replaceWith takes a plain object"