PR template: reminder to document your stuff (#4081)

* PR template: reminder to document your stuff

* link to tsdoc, not typedoc

* add full stops

.github/PULL_REQUEST_TEMPLATE.md

@@ -2,6 +2,7 @@
 
 ## Checklist
 
--   [ ] Tests written for new code (and old code if feasible)
--   [ ] Linter and other CI checks pass
--   [ ] Sign-off given on the changes (see [CONTRIBUTING.md](https://github.com/matrix-org/matrix-js-sdk/blob/develop/CONTRIBUTING.md))
+-   [ ] Tests written for new code (and old code if feasible).
+-   [ ] New or updated `public`/`exported` symbols have accurate [TSDoc](https://tsdoc.org/) documentation.
+-   [ ] Linter and other CI checks pass.
+-   [ ] Sign-off given on the changes (see [CONTRIBUTING.md](https://github.com/matrix-org/matrix-js-sdk/blob/develop/CONTRIBUTING.md)).