docs: update brats classes description (#8246)

Resolves #8225.

### Description

Updated brats18 classes description.

### 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.
- [x] Integration tests passed locally by running `./runtests.sh -f -u
--net --coverage`.
- [x] Quick tests passed locally by running `./runtests.sh --quick
--unittests --disttests`.
- [x] In-line docstrings updated.
- [x] Documentation updated, tested `make html` command in the `docs/`
folder.

Signed-off-by: Vladislav Tumko <56307628+vectorvp@users.noreply.github.com>
Co-authored-by: YunLiu <55491388+KumoLiu@users.noreply.github.com>

monai/transforms/utility/array.py

@@ -1054,12 +1054,11 @@ def __call__(
 
 class ConvertToMultiChannelBasedOnBratsClasses(Transform):
     """
-    Convert labels to multi channels based on brats18 classes:
-    label 1 is the necrotic and non-enhancing tumor core
-    label 2 is the peritumoral edema
-    label 4 is the GD-enhancing tumor
-    The possible classes are TC (Tumor core), WT (Whole tumor)
-    and ET (Enhancing tumor).
+    Convert labels to multi channels based on `brats18 <https://www.med.upenn.edu/sbia/brats2018/data.html>`_ classes,
+    which include TC (Tumor core), WT (Whole tumor) and ET (Enhancing tumor):
+    label 1 is the necrotic and non-enhancing tumor core, which should be counted under TC and WT subregion,
+    label 2 is the peritumoral edema, which is counted only under WT subregion,
+    label 4 is the GD-enhancing tumor, which should be counted under ET, TC, WT subregions.
     """
 
     backend = [TransformBackends.TORCH, TransformBackends.NUMPY]