chore(github): Improve PR template "document later" checkbox descript…

…ion (#4625)

# Description

## Problem\*

Describing the option as "Exceptional Case" wasn't very intuitive. Some
PRs that were meant to check that box checked the "No documentation
needed." box instead.

## Summary\*

Describe the option as "For Experimental Features" instead, as most if
not all such exceptional cases that docs were preferred to be delayed
came from features being still experimental (i.e. not intended for
public use yet).

## Documentation\*

Check one:
- [x] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[Exceptional Case]** Documentation to be submitted in a separate
PR.

# PR Checklist\*

- [ ] I have tested the changes locally.
- [ ] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

.github/pull_request_template.md

@@ -17,7 +17,7 @@ Resolves <!-- Link to GitHub Issue -->
 Check one:
 - [ ] No documentation needed.
 - [ ] Documentation included in this PR.
-- [ ] **[Exceptional Case]** Documentation to be submitted in a separate PR.
+- [ ] **[For Experimental Features]** Documentation to be submitted in a separate PR.
 
 # PR Checklist\*