Improve documentation of per-stream Promtail rate limiting

[KMiller-Grafana]

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.

[stale]

Hi! This issue has been automatically marked as stale because it has not had any
activity in the past 30 days.

We use a stalebot among other tools to help manage the state of issues in this project.
A stalebot can be very useful in closing issues in a number of cases; the most common
is closing issues or PRs where the original reporter has not responded.

Stalebots are also emotionless and cruel and can close issues which are still very relevant.

If this issue is important to you, please add a comment to keep it open. More importantly, please add a thumbs-up to the original issue entry.

We regularly sort for closed issues which have a stale label sorted by thumbs up.

We may also:

• Mark issues as revivable if we think it's a valid issue but isn't something we are likely
  to prioritize in the future (the issue will still remain closed).
• Add a keepalive label to silence the stalebot if the issue is very common/popular/important.

We are doing our best to respond, organize, and prioritize all issues but it can be a challenging task,
our sincere apologies if you find yourself at the mercy of the stalebot.

[cstyan]

@JStickler not sure if this is still needed, I can check docs sometime this week