DOC: document the normalized kernel in prewitt_h, prewitt_v

[grlee77]

Description

closes #5075

The documented filter kernel for prewitt_h and prewitt_v did not take the normalization of the smoothing kernel into account.

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.

[jni]

Thank you @grlee77!