Enhance documentation for explicit rowid support

Author: groue

GRDB/Documentation.docc/DatabaseSchema.md

@@ -191,14 +191,19 @@ extension Player: FetchableRecord, MutablePersistableRecord {
         AllColumns(),
         Column.rowID]
 
+    // Update id upon successful insertion
     mutating func didInsert(_ inserted: InsertionSuccess) {
         rowid = inserted.rowID
     }
 }
 
 try dbQueue.read { db in
     // SELECT *, rowid FROM player WHERE rowid = 1
-    let player = try Player.fetchOne(db, id: 1)
+    if let player = try Player.fetchOne(db, id: 1) {
+        // DELETE FROM player WHERE rowid = 1
+        let deleted = try player.delete(db)
+        print(deleted) // true
+    }
 }
 ```