Comment search works like post search with two differences.
Thread ID
Supplying a thread ID limits the search to comments under one post. This also satisfies the scope requirement, so a keyword plus a thread ID is a valid query.
Comment keyword search is slow
The archive runs a full-text query over comment bodies, which is considerably slower than searching post titles. On a busy subreddit or an active account it can time out.
If that happens, narrow the search: add a date range, reduce the limit, or restrict to one subreddit. Subdex suggests this when it detects a query likely to be slow.
Reading results
Each comment shows its subreddit, date, archived score and body. Comments marked deleted or removed are shown with that label rather than hidden, so the record stays complete.