docs(schema): add more info and examples to Schema#indexes() docs

Fix #10446

lib/schema.js

@@ -1666,8 +1666,8 @@ Object.defineProperty(Schema, 'indexTypes', {
 });
 
 /**
- * Returns a list of indexes that this schema declares, via `schema.index()`
- * or by `index: true` in a path's options.
+ * Returns a list of indexes that this schema declares, via `schema.index()` or by `index: true` in a path's options.
+ * Indexes are expressed as an array `[spec, options]`.
  *
  * ####Example:
  *
@@ -1680,6 +1680,17 @@ Object.defineProperty(Schema, 'indexTypes', {
  *     //   [ { registeredAt: 1 }, { background: true } ] ]
  *     userSchema.indexes();
  *
+ * [Plugins](/docs/plugins.html) can use the return value of this function to modify a schema's indexes.
+ * For example, the below plugin makes every index unique by default.
+ *
+ *     function myPlugin(schema) {
+ *       for (const index of schema.indexes()) {
+ *         if (index[1].unique === undefined) {
+ *           index[1].unique = true;
+ *         }
+ *       }
+ *     }
+ *
  * @api public
  * @return {Array} list of indexes defined in the schema
  */