Update postgres.rst

[HelgeKrueger]

This adds the missing documentation entry for ArrayField (postgres specific).

See #1187

[abondar]

Did you check locally that it compiles docs in a way that you expect?

[coveralls]

Pull Request Test Coverage Report for Build 11050908131

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 89.011%

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Totals
Change from base Build 11048931810:	0.0%
Covered Lines:	5965
Relevant Lines:	6586

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💛 - Coveralls

[HelgeKrueger]

It looks like. I think knowing to use element_type is what I expect to be able to use ArrayField.

Note: make doc failed with:

sphinx-build -M html docs build
Traceback (most recent call last):
  File "/home/helge/.local/bin/sphinx-build", line 5, in <module>
    from sphinx.cmd.build import main
ModuleNotFoundError: No module named 'sphinx'
make: *** [Makefile:76: docs] Error 1

I had to run poetry run sphinx-build -M html docs build.