pgsql: Doc: clarify when table rewrites happen with column additionand

Doc: clarify when table rewrites happen with column addition and DEFAULT

16828d5 has improved ALTER TABLE so as a column addition does not
require a rewrite for a non-NULL default with constant expressions, but
one spot in the documentation did not get updated consistently.
The documentation also now clarifies the fact that this does not apply
if the expression is volatile, where a table rewrite is still required.

Reported-by: Daniel Westermann
Author: Ian Barwick
Reviewed-by: Michael Paquier, Daniel Westermann
Discussion: https://postgr.es/m/DB6PR0902MB2184C7D5645CF15D75EB7957D2CF0@DB6PR0902MB2184.eurprd09.prod.outlook.com
Backpatch-through: 11

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/b990cc38dc64f3333c1c61aba247bfbaea1475f8

Modified Files
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doc/src/sgml/ddl.sgml | 34 +++++++++++++++++++++++-----------
1 file changed, 23 insertions(+), 11 deletions(-)