Add docstring for get_app #488

Job	Run time
Detect Relevant Path Changes	7s
Build Dash Package	3m 48s
Typing Tests	2m 47s
Table Server Tests (Group ${{ matrix.test-group }})	0s
Table Unit/Lint Tests (Python ${{ matrix.python-version }})	0s
Run Background & Async Callback Tests (Python ${{ matrix.python-version }})	0s
	6m 42s

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