pgsql: Clarify behavior of adding and altering a column in same ALTER c

Clarify behavior of adding and altering a column in same ALTER command.

The behavior of something like

ALTER TABLE transactions
  ADD COLUMN status varchar(30) DEFAULT 'old',
  ALTER COLUMN status SET default 'current';

is to fill existing table rows with 'old', not 'current'.  That's
intentional and desirable for a couple of reasons:

* It makes the behavior the same whether you merge the sub-commands
into one ALTER command or give them separately;

* If we applied the new default while filling the table, there would
be no way to get the existing behavior in one SQL command.

The same reasoning applies in cases that add a column and then
manipulate its GENERATED/IDENTITY status in a second sub-command,
since the generation expression is really just a kind of default.
However, that wasn't very obvious (at least not to me; earlier in
the referenced discussion thread I'd thought it was a bug to be
fixed).  And it certainly wasn't documented.

Hence, add documentation, code comments, and a test case to clarify
that this behavior is all intentional.

In passing, adjust ATExecAddColumn's defaults-related relkind check
so that it matches up exactly with ATRewriteTables, instead of being
effectively (though not literally) the negated inverse condition.
The reasoning can be explained a lot more concisely that way, too
(not to mention that the comment now matches the code, which it
did not before).

Discussion: https://postgr.es/m/10365.1558909428@sss.pgh.pa.us

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/9b9c5f279e8261ab90dc64559911d2578288b7e9

Modified Files
--------------
doc/src/sgml/ref/alter_table.sgml      | 39 ++++++++++++++++++++++++++++++----
src/backend/commands/tablecmds.c       | 20 ++++++++++-------
src/test/regress/expected/identity.out |  6 ++++++
src/test/regress/sql/identity.sql      |  6 ++++++
4 files changed, 59 insertions(+), 12 deletions(-)