Clarify documentation of DataChunkIterator

[oruebel]

Motivation

Fix #623 Clarify the expected chunk shape behavior for DataChunkIterator.

How to test the behavior?

This PR only updates docstrings

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov]

Codecov Report

Base: 87.77% // Head: 87.77% // No change to project coverage 👍

Coverage data is based on head (4f856f3) compared to base (95f1965).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #813   +/-   ##
=======================================
  Coverage   87.77%   87.77%           
=======================================
  Files          44       44           
  Lines        8936     8936           
  Branches     2061     2061           
=======================================
  Hits         7844     7844           
  Misses        777      777           
  Partials      315      315           

Impacted Files	Coverage Δ
src/hdmf/data_utils.py	90.29% <ø> (ø)

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