Some incorrect comments and out-dated README from run-time pruning

I've noticed that the comments above the PartitionedRelPruneInfo
struct incorrectly document how subplan_map and subpart_map are
indexed. This seems to have snuck in on 4e232364033.

Also, while reading the executor README file, I noticed that we
mentioned that executor nodes are created one for one for each
corresponding plan node. This is no longer completely true. Some
Append / MergeAppend subnodes may be skipped when performing run-time
pruning at executor startup. I thought it might be best to mention
that in the README.

Patch attached.

-- 
 David Rowley                   http://www.2ndQuadrant.com/
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