chore: add missing docstrings in document store base tests #12594

Job	Run time
black	49s
Install and cache ubuntu-latest dependencies	3m 25s
Install and cache macos-latest dependencies	7m 28s
Install and cache windows-latest dependencies	7m 41s
Unit / ubuntu-latest	3m 20s
Unit / windows-latest	6m 54s
Unit / macos-latest	5m 53s
Integration / ubuntu-latest	5m 41s
Integration / macos-latest	9m 54s
Integration / windows-latest	8m 26s
Catch-all check	1s
	59m 32s

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