docs/reference/builder: update "syntax" section
docs/reference/builder: update "syntax" section

- rename "experimental" to "labs"
- rephrase recommendation for picking a version
- clarify that the "labs" channel provides a superset of the "stable" channel.
- remove "External implementation features" section, because it overlapped
  with the "syntax" section.
- removed `:latest` from the "stable" channel (generally not recommended)

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

docker/cli,thaJeztah/cli,thaJeztah/cli,cpuguy83/cli,docker/cli,cpuguy83/cli