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_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/80aa66392ec8264cad6b2ad1bb442238c8771a6d

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