pgsql: Fix fuzzy thinking about amcanmulticol versus amcaninclude.

Fix fuzzy thinking about amcanmulticol versus amcaninclude.

These flags should be independent: in particular an index AM should
be able to say that it supports include columns without necessarily
supporting multiple key columns. The included-columns patch got
this wrong, possibly aided by the fact that it didn't bother to
update the documentation.

While here, clarify some text about amcanreturn, which was a little
vague about what should happen when amcanreturn reports that only
some of the index columns are returnable.

Noted while reviewing the SP-GiST included-columns patch, which
quite incorrectly (and unsafely) changed SP-GiST to claim
amcanmulticol = true as a workaround for this bug.

Backpatch to v11 where included columns were introduced.

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7c89246d0bb233be7d6670f0a8f024e99423e8cc

Modified Files
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doc/src/sgml/indexam.sgml | 35 +++++++++++++++++++++++++++--------
src/backend/commands/indexcmds.c | 4 ++--
2 files changed, 29 insertions(+), 10 deletions(-)