Add doc to H5DataIO.dataset setter

[rly]

Motivation

While reading through the code for H5DataIO, I was confused about the purpose of the dataset property and how it differs from data since it is not used locally:

hdmf/src/hdmf/backends/hdf5/h5_utils.py

Lines 545 to 555 in dfb1df7

	self.__dataset = None
	@property
	def dataset(self):
	return self.__dataset
	@dataset.setter
	def dataset(self, val):
	if self.__dataset is not None:
	raise ValueError("Cannot overwrite H5DataIO.dataset")
	self.__dataset = val

It turns out dataset was added for a very specific use case where a user wants to create an empty hdf5 dataset, write it to disk, and have access to the handle to that dataset without having to close and reopen the file -- #747

To prevent myself and others from retracing those steps to understand this code, I added a docstring.

Checklist

• Did you update CHANGELOG.md with your changes?
• Does the PR clearly describe the problem and the solution?
• Have you reviewed our Contributing Guide?
• Does the PR use "Fix #XXX" notation to tell GitHub to close the relevant issue numbered XXX when the PR is merged?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.83%. Comparing base (dfb1df7) to head (b95ae85).
Report is 27 commits behind head on dev.

Additional details and impacted files

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##              dev    #1154   +/-   ##
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  Coverage   88.83%   88.83%           
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  Files          45       45           
  Lines        9784     9784           
  Branches     2779     2779           
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  Hits         8692     8692           
  Misses        776      776           
  Partials      316      316           

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