Document how to set replication methods and keys for tables on DB taps using metadata extra

Going beyond https://meltano.com/docs/integration.html#setting-metadata, we should document how to use the metadata extractor extra specifically to set replication-method and replication-key for DB taps like tap-mongodb and the PipelineWise DB taps we'll add as part of #2357 (closed): #2410 (closed) #2411 (closed)