Improve docs for "isotropic" arg in RandomAffine (#1163)

* Detailed documentation for isotropic argument in random affine transform

As discussed in #1158, additinal documentation is added to prevent confusion.

* flake8 line too long resolved

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Update src/torchio/transforms/augmentation/spatial/random_affine.py

Co-authored-by: Fernando Pérez-García <[email protected]>

* Update src/torchio/transforms/augmentation/spatial/random_affine.py

Co-authored-by: Fernando Pérez-García <[email protected]>

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Fernando Pérez-García <[email protected]>

src/torchio/transforms/augmentation/spatial/random_affine.py

@@ -68,8 +68,12 @@ class RandomAffine(RandomTransform, SpatialTransform):
  10 mm to the right, 20 mm to the front, and 30 mm upwards.
  If the image was in, e.g., PIR+ orientation, the sample will move
  10 mm to the back, 20 mm downwards, and 30 mm to the right.
- isotropic: If ``True``, the scaling factor along all dimensions is the
- same, i.e. :math:`s_1 = s_2 = s_3`.
+ isotropic: If ``True``, only one scaling factor will be sampled for all dimensions,
+ i.e. :math:`s_1 = s_2 = s_3`.
+ If one value :math:`x` is provided in :attr:`scales`, the scaling factor along all
+ dimensions will be :math:`s \sim \mathcal{U}(1 - x, 1 + x)`.
+ If two values provided :math:`(a, b)` in :attr:`scales`, the scaling factor along all
+ dimensions will be :math:`s \sim \mathcal{U}(a, b)`.
  center: If ``'image'``, rotations and scaling will be performed around
  the image center. If ``'origin'``, rotations and scaling will be
  performed around the origin in world coordinates.
@@ -442,7 +446,9 @@ def _parse_scales_isotropic(scales, isotropic):
  if isotropic and len(scales) in (3, 6):
  message = (
  'If "isotropic" is True, the value for "scales" must have'
- f' length 1 or 2, but "{scales}" was passed'
+ f' length 1 or 2, but "{scales}" was passed.'
+ ' If you want to set isotropic scaling, use a single value or two values as a range'
+ ' for the scaling factor. Refer to the documentation for more information.'
  )
  raise ValueError(message)