Update the user guide to use channel_axis rather than multichannel

[grlee77]

Description

closes #5554

Updates the user guide to better describe the more flexible channel support via channel_axis. Also, avoids use of the deprecated multichannel argument in examples.

The description of img_as_float was also updated slightly to indicate its current behavior (no longer automatically rescales floating point inputs and preserves floating point precision).

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]

Thanks for taking care of this!

[grlee77]

Updated. Thanks for the helpful suggestions, @mkcor.

[grlee77]

The linux-cp3.8-pre failure here is unrelated (appears to be an issue with a new setuptools_scm release).

It looks like upstream has already fixed it in: pypa/setuptools_scm#628