Documentation for "isotropic" in random affine transform

[AminAlam]

As discussed in #1158, additinal documentation is added to prevent confusion when "isotropic" is true and "scales" has more than 2 elements.

Fixes #1158.

Description

Checklist

• [x] I have read the CONTRIBUTING docs and have a developer setup (especially important are pre-commitand pytest)
• [x] Non-breaking change (would not break existing functionality)
• [ ] Breaking change (would cause existing functionality to change)
• [ ] Tests added or modified to cover the changes
• [x] Integration tests passed locally by running pytest
• [x] In-line docstrings updated
• [x] Documentation updated, tested running make html inside the docs/ folder
• [x] This pull request is ready to be reviewed