+ * Joins using parentheses for grouping (since MediaWiki 1.31) may be
+ * constructed using nested arrays. For example,
+ *
+ * [ 'tableA', 'nestedB' => [ 'tableB', 'b2' => 'tableB2' ] ]
+ *
+ * along with `$join_conds` like
+ *
+ * [ 'b2' => [ 'JOIN', 'b_id = b2_id' ], 'nestedB' => [ 'LEFT JOIN', 'b_a = a_id' ] ]
+ *
+ * will produce SQL something like
+ *
+ * FROM tableA LEFT JOIN (tableB JOIN tableB2 AS b2 ON (b_id = b2_id)) ON (b_a = a_id)
+ *