Tweak image upload help text (#3774)

A user mentioned some confusion by clinicians trying to upload dicoms.
Imho the user had a valid point. The help text also had the suggestion
that .tiff was converted to MHA. That is incorrect. I opted to sacrifice
some horizontal space for clarity.

## Before


![image](https://github.com/user-attachments/assets/8dfde665-f2da-493f-8bd4-ff39b6fde567)

## After


![image](https://github.com/user-attachments/assets/3c2ef64a-d1ff-44a5-bf84-5db5ca8057e2)

Note: text is used in a widget, no need for a migration.

app/grandchallenge/cases/forms.py

@@ -8,20 +8,20 @@
 from grandchallenge.uploads.models import UserUpload
 from grandchallenge.uploads.widgets import UserUploadMultipleWidget
 
-IMAGE_UPLOAD_HELP_TEXT = (
-    "The total size of all files uploaded in a single session "
-    "cannot exceed 10 GB. "
-    "A maximum of 100 files can be uploaded per session. "
-    "Please only upload one volume per session. "
-    "If your volume consists of more than this then please compress the "
-    "directory and upload a single .zip file.<br>"
-    "The following file formats are supported and will be converted to MHA: "
-    ".mha, .mhd, .raw, .zraw, .dcm, .nii, .nii.gz, .nrrd, "
-    ".fda, .fds, .tiff, .png, .jpeg and .jpg.<br>"
-    "The following file formats can be uploaded and will be converted to "
-    "TIFF: Aperio (.svs), Hamamatsu (.vms, .vmu, .ndpi), Leica (.scn), "
-    "MIRAX (.mrxs), Ventana (.bif) and DICOM-WSI (.dcm)."
-)
+IMAGE_UPLOAD_HELP_TEXT = """
+The total size of all files uploaded in a single session cannot exceed 10 GB.
+A maximum of 100 files can be uploaded per session. Please only upload one volume per session.
+<br>
+If your volume is made up of multiple individual slices (e.g. DICOM files), please
+compress the entire directory into a single .zip file and upload that file.
+<br>
+The following file formats are supported and will be converted to MHA format:
+.mha, .mhd, .raw, .zraw, .dcm, .nii, .nii.gz, .nrrd, .fda, .fds, .png, .jpeg, and .jpg.
+<br>
+The following file formats can be uploaded and will be converted to TIF format:
+.tiff, Aperio (.svs), Hamamatsu (.vms, .vmu, .ndpi), Leica (.scn), MIRAX (.mrxs),
+Ventana (.bif), and DICOM-WSI (.dcm).
+"""
 
 
 class UploadRawImagesForm(SaveFormInitMixin, forms.ModelForm):