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.

### Checklist
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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: cdc858cd88fbf86661662147210f9587117aa593

Change-Id: I3f2c8fe346d3235fa8ba12c4d9ab712ddb840230