Add documentation for the OP_SorterInsert opcode, formerly omitted by mistake.

No changes to code.

FossilOrigin-Name: 16d88a907730e3773a1320dbaf1f82c2bc71d71f

src/vdbe.c

@@ -5038,6 +5038,13 @@ next_tail:
 ** This instruction only works for indices.  The equivalent instruction
 ** for tables is OP_Insert.
 */
+/* Opcode: SorterInsert P1 P2 * * *
+** Synopsis: key=r[P2]
+**
+** Register P2 holds an SQL index key made using the
+** MakeRecord instructions.  This opcode writes that key
+** into the sorter P1.  Data for the entry is nil.
+*/
 case OP_SorterInsert:       /* in2 */
 case OP_IdxInsert: {        /* in2 */
   VdbeCursor *pC;