doc: expand description of how non-SELECT queries are processed

doc:  expand description of how non-SELECT queries are processed

The previous description of how the executor processes non-SELECT
queries was very dense, causing lack of clarity.  This expanded text
spells it out more simply.

Reported-by: fotis.koutoupas@gmail.com
Discussion: https://postgr.es/m/160912275508.676.17469511338925622905@wrigleys.postgresql.org

Backpatch-through: 9.5