After merge in of #5051, which adds a new per-stream rate-limiting pipeline stage to Promtail, improve these items:

• The match pipeline stage schema needs a new stages entry for <limit_stage>.

• The description of the limit stage can become more clear to readers if it identifies that it is implementing rate limits on a per stream basis. The examples probably should include YAML for the match stage, and describe/state that it defines the stream that the rate limit is applied to.