Docstring improvement for ConvertToMultiChannelBasedOnBratsClasses

[mingxin-zheng]

Describe the bug

The docstring below was confusing to read for people who haven't worked on BRATS 18 before (e.g. me)

MONAI/monai/transforms/utility/array.py

Line 1054 in 13b96ae

Convert labels to multi channels based on brats18 classes:

I would suggest to elaborate it as below:

Convert labels to multi channels based on brats18 classes, which include TC (Tumor core), WT (Whole tumor) and ET (Enhancing tumor) in order.
According to the [official page](https://www.med.upenn.edu/sbia/brats2018/data.html)
    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 subresion
    label 4 is the GD-enhancing tumor, which should be counted under ET, TC, WT subregions

Expected behavior
A clear and concise description of the function behavior and expected outputs

Additional context
Related issue: Project-MONAI/tutorials#1355
Related comment: Project-MONAI/VLM#55 (comment)