residual multichannel->channel_axis fixes

[grlee77]

Description

The first commit here cleans up a few residual uses of multichannel in docstring examples, parameter descriptions and warning texts.

The second commit is to update the benchmark scripts with a _channel_kwarg helper that will specify multichannel when benchmarking against skimage<0.19, but channel_axis on newer versions.

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

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]

Yay! Thank you, @grlee77 🙏

[rfezzani]

🎉 Thank you @grlee77 for your hard work and @mkcor for the reminder 😉