chore: refine PR template (#3074)

given that we now use the PR description as the commit message, the PR
template should point that out. Also, a `# Summary` is relatively
strange in a commit message, so removed it.

---------

Co-authored-by: Scott Morrison <scott.morrison@gmail.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,13 +1,14 @@
-# Read and remove this section before submitting
+# Read this section before submitting
 
 * Ensure your PR follows the [External Contribution Guidelines](https://github.com/leanprover/lean4/blob/master/CONTRIBUTING.md).
 * Please make sure the PR has excellent documentation and tests. If we label it `missing documentation` or `missing tests` then it needs fixing!
-* Add the link to your `RFC` or `bug` issue below.
+* Include the link to your `RFC` or `bug` issue in the description.
 * If the issue does not already have approval from a developer, submit the PR as draft.
-* Remove this section before submitting.
+* The PR title/description will become the commit message. Keep it up-to-date as the PR evolves.
+* If you rebase your PR onto `nightly-with-mathlib` then CI will test Mathlib against your PR.
+* You can manage the `awaiting-review`, `awaiting-author`, and `WIP` labels yourself, by writing a comment containing one of these labels on its own line.
+* Remove this section, up to and including the `---` before submitting.
 
-You can manage the `awaiting-review`, `awaiting-author`, and `WIP` labels yourself, by writing a comment containing one of these labels on its own line.
+---
 
-# Summary
-
-Link to `RFC` or `bug` issue:
+Closes #0000 (`RFC` or `bug` issue number fixed by this PR, if any)