put back sections for custom sign-in and sign-up flows

docs/references/react/use-sign-in.mdx

@@ -110,3 +110,7 @@ The `status` property of the `SignIn` object can be one of the following values:
 
   The user needs to set a new password.
 </Properties>
+
+### Create a custom sign-in flow with `useSignIn()`
+
+The `useSignIn()` hook can also be used to build fully custom sign-in flows, if Clerk's prebuilt components don't meet your specific needs or if you require more control over the authentication flow. Different sign-in flows include email and password, email and phone codes, email links, and multifactor (MFA). To learn more about using the `useSignIn()` hook to create custom flows, see the [custom flow guides](/docs/custom-flows/overview).

docs/references/react/use-sign-up.mdx

@@ -96,3 +96,7 @@ The `status` property of the `SignUp` object can be one of the following values:
 
   A requirement is missing from the [Email, Phone, Username](https://dashboard.clerk.com/last-active?path=user-authentication/email-phone-username) settings. For example, in the Clerk Dashboard, the **Password** setting is required but a password wasn't provided in the custom flow.
 </Properties>
+
+### Create a custom sign-up flow with `useSignUp()`
+
+The `useSignUp()` hook can also be used to build fully custom sign-up flows, if Clerk's prebuilt components don't meet your specific needs or if you require more control over the authentication flow. Different sign-up flows include email and password, email and phone codes, email links, and multifactor (MFA). To learn more about using the `useSignUp()` hook to create custom flows, see the [custom flow guides](/docs/custom-flows/overview).