[python] Add missing lifecycle methods and expand docstrings

[thetorpedodog]

• Adds mode, close, and closed methods/properties.
• Fills in most of the docstrings with details about parameters and expected behavior.

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This change depends upon the result of https://github.com/single-cell-data/TileDB-SOMA/pull/910.

Part of #107 and https://github.com/single-cell-data/TileDB-SOMA/issues/638.