Update user warning message for viewer module.

[mkcor]

Description

As per #6119 (comment)

Reference: https://github.com/scikit-image/scikit-image/discussions/5651#discussioncomment-1596365

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8
• Descriptive commit messages (see below)

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.

[mkcor]

And I guess all TODO's in this module could/should be converted into WON'TDO's?

mkcor@[...]scikit-image/skimage/viewer$ ack TODO
utils/dialogs.py
31:    # TODO: io plugins should assign default image formats

plugins/overlayplugin.py
39:        #TODO: `color` doesn't update GUI widget when set manually.

plugins/plotplugin.py
49:        #TODO: Converted color is slightly different than Qt background.

plugins/base.py
68:    #TODO: Add flag so image is not passed to filter function by default.
123:        #TODO: Always passing image as first argument may be bad assumption.

widgets/history.py
89:            # TODO: This check/conversion should probably be in `imsave`.

viewers/core.py
103:        # TODO: Add ImageViewer to skimage.io window manager
376:        # TODO: Adjust height to accommodate slider; the following doesn't work

[hmaarrfk]

@meeseeksdev backport to v0.19.x