pgsql: 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

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/931c0bda2dd77781e0efdae47dda5ec478fde083

Modified Files
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doc/src/sgml/arch-dev.sgml | 50 +++++++++++++++++++++++++++-------------------
1 file changed, 30 insertions(+), 20 deletions(-)