Doc: add detail about EXPLAIN's "Disabled" property

Doc: add detail about EXPLAIN's "Disabled" property

c01743aa4 and later 161320b4b adjusted the EXPLAIN output to show which
plan nodes were chosen despite being disabled by the various enable*
GUCs.  Prior to e22253467, the disabledness of a node was only evident by
a large startup cost penalty.  Since we now explicitly tag disabled nodes
with a boolean property in EXPLAIN, let's add some documentation to
provide some details about why and when disabled nodes can appear in the
plan.

Author: Laurenz Albe, David Rowley
Discussion: https://postgr.es/m/883729e429267214753d5e438c82c73a58c3db5d.camel@cybertec.at