Activities: Add instructions for Source Type selection

#### Describe the task

To ensure users select at least one source type from the list, add instructional text for each activity with the following:

- [ ] Please select all applicable source types for this activity

#### Acceptance Criteria

- [ ] first
- [ ] second
- [ ] third

#### Definition of Done

- [ ] first
- [ ] second
- [ ] third

#### Additional context

- Add any other context about the task here.
- Or here

Start by locating the activity form and the existing Source Type selection UI. Add “Please select all applicable source types for this activity” for each activity, then verify that the instruction appears consistently and that the source type choices remain usable. The issue does not name a file or test, so some repository exploration is needed.

Written by the indexing model from the issue text.