docs: advanced field includes the option to generateId (#1756)

docs/content/docs/reference/options.mdx

@@ -371,7 +371,14 @@ export const auth = betterAuth({
 			httpOnly: true,
 			secure: true
 		},
-		cookiePrefix: "myapp"
+		cookiePrefix: "myapp",
+		generateId: ((options: {
+			model: LiteralUnion<Models, string>;
+			size?: number;
+		}) => {
+			// Generate a unique ID for the model
+			return "my-id";
+		}) 
 	},
 })
 ```
@@ -383,6 +390,7 @@ export const auth = betterAuth({
 - `cookies`: Customize cookie names and attributes
 - `defaultCookieAttributes`: Default attributes for all cookies
 - `cookiePrefix`: Prefix for cookies
+- `generateId`: Function to generate a unique ID for a model
 
 ## `databaseHooks`