update round trip docs

[bendichter]

update round trip testing docs to the new NWBH5IOMixin and Acquisitio…nH5IOMixin classes

Motivation

fix #1585

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

Merging #1586 (09b01fe) into dev (b5180c3) will increase coverage by 6.90%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##              dev    #1586      +/-   ##
==========================================
+ Coverage   84.44%   91.34%   +6.90%     
==========================================
  Files          25       25              
  Lines        2507     2507              
  Branches      471      471              
==========================================
+ Hits         2117     2290     +173     
+ Misses        307      137     -170     
+ Partials       83       80       -3     

Flag	Coverage Δ
integration	70.56% <ø> (?)
unit	84.44% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/pynwb/file.py	87.93% <0.00%> (+0.25%)	⬆️
src/pynwb/__init__.py	76.15% <0.00%> (+0.76%)	⬆️
src/pynwb/core.py	76.13% <0.00%> (+1.13%)	⬆️
src/pynwb/ophys.py	95.65% <0.00%> (+1.44%)	⬆️
src/pynwb/icephys.py	91.96% <0.00%> (+2.25%)	⬆️
src/pynwb/misc.py	91.24% <0.00%> (+3.64%)	⬆️
src/pynwb/io/file.py	94.33% <0.00%> (+11.94%)	⬆️
src/pynwb/io/core.py	79.62% <0.00%> (+18.51%)	⬆️
src/pynwb/io/base.py	85.05% <0.00%> (+26.43%)	⬆️
src/pynwb/io/image.py	100.00% <0.00%> (+33.33%)	⬆️
... and 3 more

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