add queue limit config comments in gitaly config

In gitlab-org/gitaly!4270 (merged) and gitlab-org/gitaly!4271 (merged), we added config values to control how large the concurrency queue in gitaly can grow to, as well as how long a request can wait in the queue.

Default is 0 for both, which is a noop in Gitaly. Add comments in the config toml so customers are aware these config values exist.

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Integration tests added to GitLab QA
• The impact any change in container size has should be evaluated