Add a note asking for headers or documentation in PRs

Having a link to relevant source makes PRs much easier to review,
especially on less common platforms. Request that as part of the PR
template.

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,15 +1,25 @@
 Thanks for considering submitting a PR!
 
-We have the [contribution guide](https://github.com/rust-lang/libc/blob/main/CONTRIBUTING.md). Please read it if you're new here!
+We have the
+[contribution guide](https://github.com/rust-lang/libc/blob/main/CONTRIBUTING.md).
+Please read it if you're new here!
 
-Here's a checklist for things that will be checked during review or continuous integration.
+Here is a checklist for things that will be checked during review or continuous
+integration:
 
-- \[ ] Edit corresponding file(s) under `libc-test/semver` when you add/remove item(s), e.g. edit `linux.txt` if you add an item to `src/unix/linux_like/linux/mod.rs`
-- \[ ] Your PR doesn't contain any private or *unstable* values like `*LAST` or `*MAX` (see [#3131](https://github.com/rust-lang/libc/issues/3131))
+- \[ ] Edit corresponding file(s) under `libc-test/semver` when you add/remove
+  item(s), e.g. edit `linux.txt` if you add an item to
+  `src/unix/linux_like/linux/mod.rs`
+- \[ ] Your PR doesn't contain any private or _unstable_ values like `*LAST` or
+  `*MAX` (see [#3131](https://github.com/rust-lang/libc/issues/3131))
+- \[ ] Provide a link to relevant source (headers or documentation) if your PR
+  adds or changes API.
 - \[ ] If your PR has a breaking change, please clarify it
-- \[ ] If your PR increments version number, it must NOT contain any other changes (otherwise a release could be delayed)
+- \[ ] If your PR increments version number, it must NOT contain any other
+  changes (otherwise a release could be delayed)
 - \[ ] Make sure `ci/style.sh` passes
 - \[ ] `cd libc-test && cargo test`
-  - (this might fail on your env due to environment difference between your env and CI. Ignore local failures if you are not sure)
+  - (this might fail on your env due to environment difference between your env
+    and CI. Ignore local failures if you are not sure)
 
 Delete this line and everything above before opening your PR.