document postgresql_nulls_not_distinct

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### Description
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https://github.com/sqlalchemy/sqlalchemy/issues/8240 and https://github.com/sqlalchemy/sqlalchemy/pull/9834 added support for `NULLS NOT DISTINCT` to the PostgreSQL dialect, but didn't add it to the docs (only the change log). This adds a section to the "Constraint Options" section of the PostgreSQL dialect docs.

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This pull request is:

- [x] A documentation / typographical / small typing error fix
	- Good to go, no issue or tests are needed
- [ ] A short code fix
	- please include the issue number, and create an issue if none exists, which
	  must include a complete example of the issue.  one line code fixes without an
	  issue and demonstration will not be accepted.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.   one line code fixes without tests will not be accepted.
- [ ] A new feature implementation
	- please include the issue number, and create an issue if none exists, which must
	  include a complete example of how the feature would look.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.

**Have a nice day!**

Closes: #13279
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/13279
Pull-request-sha: cdc858cd88

Change-Id: I3f2c8fe346d3235fa8ba12c4d9ab712ddb840230

doc/build/changelog/changelog_20.rst

@@ -4118,7 +4118,7 @@
         :tags: usecase, postgresql
         :tickets: 8240
 
-        Added support for PostgreSQL 10 ``NULLS NOT DISTINCT`` feature of
+        Added support for PostgreSQL 15 ``NULLS NOT DISTINCT`` feature of
         unique indexes and unique constraint using the dialect option
         ``postgresql_nulls_not_distinct``.
         Updated the reflection logic to also correctly take this option

lib/sqlalchemy/dialects/postgresql/base.py

@@ -1528,6 +1528,29 @@ database to use its default of ``VIRTUAL``; on PostgreSQL 17 and earlier,
 
 
 
+``NULLS NOT DISTINCT``
+^^^^^^^^^^^^^^^^^^^^^^
+
+By default, two ``null`` values are not considered equal for unique constraints
+and indexes. Therefore, seemingly duplicate rows may be stored if one of the
+values in the constraint is ``null``. This default behavior is implementation
+defined, so other SQL dialects may behave differently than PostgreSQL.
+
+The ``NULLS NOT DISTINCT`` clause can be used to change this behavior, treating
+null values as equal and preventing unintended duplicate rows. The opposite
+``NULLS DISTINCT`` clause can also be used to make PostgreSQL's default behavior
+explict.
+
+The ``postgresql_nulls_not_distinct`` parameter can be set to ``True`` to
+add the ``NULLS NOT DISTINCT`` clause, or ``False`` to add ``NULLS DISTINCT``.
+Not setting it, or passing ``None``, will not add a clause and keep the default
+behavior.
+
+This feature requires PostgreSQL 15 or later.
+
+.. versionadded:: 2.0.16
+
+
 .. _postgresql_table_valued_overview:
 
 Table values, Table and Column valued functions, Row and Tuple objects