Issue #2949378 by masipila, heddn, phenaproxima: Document that SqlBase source...

 * - target: (optional) The database target name. Defaults to 'default'.

 * - batch_size: (optional) Number of records to fetch from the database during

 *   each batch. If omitted, all records are fetched in a single query.

 * - ignore_map: (optional) Source data is joined to the map table by default.

 *   If set to TRUE, the map table will not be joined.

 * - ignore_map: (optional) Source data is joined to the map table by default to

 *   improve migration performance. If set to TRUE, the map table will not be

 *   joined. Using expressions in the query may result in column aliases in the

 *   JOIN clause which would be invalid SQL. If you run into this, set

 *   ignore_map to TRUE.

 *

 * For other optional configuration keys inherited from the parent class, refer

 * to \Drupal\migrate\Plugin\migrate\source\SourcePluginBase.