docs: add jsonSchema field

docs/site/Model.md

@@ -215,6 +215,30 @@ The complete list of valid attributes for property definitions can be found in
 LoopBack 3's
 [Model definition section](https://loopback.io/doc/en/lb3/Model-definition-JSON-file.html#properties).
 
+You can also specify the validation rules in the field `jsonSchema`. For
+example:
+
+```ts
+@model()
+class Product extends Entity {
+  @property({
+    name: 'name',
+    description: "The product's common name.",
+    type: 'string',
+    // Specify the JSON validation rules here
+    jsonSchema: {
+      maxLength: 30,
+      minLength: 10,
+    },
+  })
+  public name: string;
+}
+```
+
+Check out the documentation of
+[Parsing requests](Parsing-requests.md#request-body) to see how to do it in
+details.
+
 <!-- NOTE(kjdelisle): Until we have a metadata docs section, link to the
 package in the repository. -->
 

docs/site/Parsing-requests.md

@@ -179,8 +179,32 @@ in/by the `@requestBody` decorator. Please refer to the documentation on
 [@requestBody decorator](Decorators.md#requestbody-decorator) to get a
 comprehensive idea of defining custom validation rules for your models.
 
-We support `json`, `urlencoded`, and `text` content types. The client should set
-`Content-Type` http header to `application/json`,
+You can also specify the JSON schema validation rules in the model property
+decorator. The rules are added in a field called `jsonSchema`, like:
+
+```ts
+@model()
+class Product extends Entity {
+  @property({
+    name: 'name',
+    description: "The product's common name.",
+    type: 'string',
+    // Specify the JSON validation rules here
+    jsonSchema: {
+      maxLength: 30,
+      minLength: 10,
+    },
+  })
+  public name: string;
+}
+```
+
+A full list of validation keywords could be found in the
+[documentation of AJV validation keywords](https://github.com/epoberezkin/ajv#validation-keywords).
+
+One request body specification could contain multiple content types. Our
+supported content types are `json`, `urlencoded`, and `text`. The client should
+set `Content-Type` http header to `application/json`,
 `application/x-www-form-urlencoded`, or `text/plain`. Its value is matched
 against the list of media types defined in the `requestBody.content` object of
 the OpenAPI operation spec. If no matching media types is found or the type is