docs: clarify backward mapping logic for RandAffine and RandAffined (#8730)

Fixes #8726 

### Description

- Updated `monai/transforms/spatial/array.py`: Added a Note to
`RandAffine `explaining the inverse relationship for translation and
scaling.
- Updated `monai/transforms/spatial/dictionary.py`: Added a
corresponding Note to `RandAffined `to ensure consistency for
dictionary-based workflows.

### Types of changes
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [x] Non-breaking change (fix or new feature that would not break
existing functionality).
- [ ] Breaking change (fix or new feature that would cause existing
functionality to change).
- [ ] New tests added to cover the changes.
- [ ] Integration tests passed locally by running `./runtests.sh -f -u
--net --coverage`.
- [ ] Quick tests passed locally by running `./runtests.sh --quick
--unittests --disttests`.
- [ ] In-line docstrings updated.
- [ ] Documentation updated, tested `make html` command in the `docs/`
folder.

Signed-off-by: ytl0623 <david89062388@gmail.com>
Co-authored-by: Eric Kerfoot <17726042+ericspod@users.noreply.github.com>

Author: ytl0623

monai/transforms/spatial/array.py

@@ -2436,6 +2436,13 @@ def __init__(
             - :py:class:`RandAffineGrid` for the random affine parameters configurations.
             - :py:class:`Affine` for the affine transformation parameters configurations.
 
+        Note:
+            The affine transformations in MONAI use a 'backward mapping' (image-to-grid) logic.
+            This can be counter-intuitive:
+            - Translation: A positive value shifts the image in the negative direction.
+            - Scaling: Positive scale_range values decrease the image size; values in [-1, 0) increase it.
+            - Rotation: The direction (CW/CCW) may vary depending on the axis.
+
         """
         RandomizableTransform.__init__(self, prob)
         LazyTransform.__init__(self, lazy=lazy)

monai/transforms/spatial/dictionary.py

@@ -1108,6 +1108,13 @@ def __init__(
             - :py:class:`monai.transforms.compose.MapTransform`
             - :py:class:`RandAffineGrid` for the random affine parameters configurations.
 
+        Note:
+            The affine transformations in MONAI use a 'backward mapping' (image-to-grid) logic.
+            This can be counter-intuitive:
+            - Translation: A positive value shifts the image in the negative direction.
+            - Scaling: Positive scale_range values decrease the image size; values in [-1, 0) increase it.
+            - Rotation: The direction (CW/CCW) may vary depending on the axis.
+
         """
         MapTransform.__init__(self, keys, allow_missing_keys)
         RandomizableTransform.__init__(self, prob)