Doc: rearrange high-level commentary about node support coverage.
authorTom Lane <tgl@sss.pgh.pa.us>
Sat, 9 Jul 2022 19:10:15 +0000 (15:10 -0400)
committerTom Lane <tgl@sss.pgh.pa.us>
Sat, 9 Jul 2022 19:10:15 +0000 (15:10 -0400)
commit3cd0ac987819280eb8dd3e0997f0294b8bc6355a
treeadcb32ec90bca7d80e16bbd4e2cbf2d4242d4a85
parent8c73c11a0d39049de2c1f400d8765a0eb21f5228
Doc: rearrange high-level commentary about node support coverage.

copyfuncs.c and friends no longer seem like great places to put
high-level remarks about what's covered and what isn't.  Move that
material to backend/nodes/README and other more-prominent places.
Add back (versions of) some remarks that disappeared in 2be87f092.

Discussion: https://postgr.es/m/3843645.1657385930@sss.pgh.pa.us
src/backend/nodes/README
src/backend/nodes/outfuncs.c
src/backend/nodes/readfuncs.c
src/include/nodes/execnodes.h