Update and extend the EXPLAIN-related documentation.

I've made a significant effort at filling in the "Using EXPLAIN" section
to be reasonably complete about mentioning everything that EXPLAIN can
output, including the "Rows Removed" outputs that were added by Marko
Tiikkaja's recent documentation-free patch.  I also updated the examples to
be consistent with current behavior; several of them were not close to what
the current code will do.  No doubt there's more that can be done here, but
I'm out of patience for today.