[py] update execute_script and execute_async_script docstring for usages

Author: lmtierney

py/selenium/webdriver/remote/webdriver.py

@@ -612,7 +612,7 @@ def execute_script(self, script, *args):
          - \*args: Any applicable arguments for your JavaScript.
 
         :Usage:
-            driver.execute_script('document.title')
+            driver.execute_script('return document.title;')
         """
         converted_args = list(args)
         command = None
@@ -634,7 +634,9 @@ def execute_async_script(self, script, *args):
          - \*args: Any applicable arguments for your JavaScript.
 
         :Usage:
-            driver.execute_async_script('document.title')
+            script = "var callback = arguments[arguments.length - 1]; " \
+                     "window.setTimeout(function(){ callback('timeout') }, 3000);"
+            driver.execute_async_script(script)
         """
         converted_args = list(args)
         if self.w3c: