Stale documentation about UniqueOpts #1178

Currently, documentation about UniqueOpts is stale for ByState condition:

	// Warning: Removing any states from the default list (other than `retryable`)
	// forces a fallback to a slower insertion path that takes an advisory lock
	// and performs a look up before insertion. This path is deprecated and should
	// be avoided if possible.

After reading this and setting my list to:

				rivertype.JobStateAvailable,
				rivertype.JobStateRetryable,
				rivertype.JobStateScheduled,

I got an error that pending and running is missing. Looking at the code it looks like this deprecated path has been removed in #614, but docstrings has not been updated.

BTW, I am sad that it is not possible to remove "running" state because now I cannot encode my use case. I have a bridge indexing data from PostgreSQL to ElasticSearch and in some cases I have to index additional documents. So I write their IDs into a table and submit a job to a single worker queue which then loops and reads from the table and process IDs in there until they are all indexed. So during this processing, more document IDs might be written into a table. Maybe current job will pick it up or maybe current job will skip it. This is why I want that it is possible to schedule additional but only one more job after currently running, so that after current job finishes, another one tries to do more work. So I would like to have ByState without "running" option.