[Extensions] Use Promise based language in JSON schemas.

Updates JSON schemas in `extensions/common/api/` to use Promise based
language for the `returns_async.description` field. This matches the
Promise API signatures that are given priority in the documentation
where these methods are shown.

Bug: N/A
Change-Id: I04417f28e56c8dee3273029348cd85d78313a558
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7313961
Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Commit-Queue: Oliver Dunk <oliverdunk@chromium.org>
Reviewed-by: Tim <tjudkins@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1565518}