Doc updates for MERGE

Attached is a patch fixing a few doc omissions for MERGE.

I don't think that it's necessary to update every place that could
possibly apply to MERGE, but there are a few places where we give a
list of commands that may be used in a particular context, and I think
those should mention MERGE, if it applies.

Also, there were a couple of other places where it seemed worth giving
slightly more detail about what happens for MERGE.

Regards,
Dean