'use client' - flesh out usages and reference (#6362)

Co-authored-by: Soichiro Miki <smiki-tky@umin.ac.jp>
Co-authored-by: David McCabe <davemccabe@gmail.com>
Co-authored-by: Luna Wei <luwe@fb.com>

Author: 4 people

public/images/docs/diagrams/use_client_module_dependency.dark.png

@@ -19,5 +19,5 @@ Directives provide instructions to [bundlers compatible with React Server Compon
 
 ## Source code directives {/*source-code-directives*/}
 
-* [`'use client'`](/reference/react/use-client) marks source files whose components execute on the client.
+* [`'use client'`](/reference/react/use-client) lets you mark what code runs on the client.
 * [`'use server'`](/reference/react/use-server) marks server-side functions that can be called from client-side code.