docs: document distribution parameters and fix stale scalar rotation example

[laurapiro17]

Description

small docs-only change to the random-parameter docs:

• the augmentation tutorial showed tio.Affine(degrees=15) with the comment "rotate uniformly between -15° and 15°", but in v2 a scalar is deterministic (it always rotates 15°). changed the example to degrees=(-15, 15) so it matches the comment.
• Distribution is listed as a valid parameter spec in the reference and mentioned in the tutorial and migration guide, but it was never shown anywhere. added a ## Distribution section to reference/random_parameters.md (parallel to ## Choice) plus a short example in the tutorial, covering both broadcasting and per-axis mixing.

checked that every snippet runs against the current code.

Checklist

• I have read the CONTRIBUTING docs and have a developer setup ready
• Changes are
  • Non-breaking (would not break existing functionality)
  • Breaking (would cause existing functionality to change)
• Tests added or modified to cover the changes
• In-line docstrings updated
• Documentation updated
• This pull request is ready to be reviewed