Rework XML documentation for WordPress.PHP.RestrictedPHPFunctions

Some of these changes were suggested during the review of PR 2491,
and others were decided while working on the new PR:
- Make the standard description generic instead of mentioning
  create_function() specifically, following the pattern used by other
  docs like DeprecatedFunctionsStandard.xml.
- Use "must not" instead of "should not" since the sniff produces an
  error.
- Simplify the code examples by removing the add_action() wrapper.
- Add <em> tags to the valid code example.

Author: rodrigoprimo

WordPress/Docs/PHP/RestrictedPHPFunctionsStandard.xml

@@ -5,24 +5,23 @@
     >
     <standard>
     <![CDATA[
-    The restricted function create_function() should not be used.
-
-    create_function() is deprecated as of PHP 7.2 and removed in PHP 8.0. Please use declared named or anonymous functions instead.
+    Certain PHP functions must not be used. Use the recommended alternatives instead.
     ]]>
     </standard>
     <code_comparison>
-        <code title="Valid: Using anonymous function (closure).">
+        <code title="Valid: Using an anonymous function.">
         <![CDATA[
-add_action( 'init', function() {
+$function = <em>function ()</em> {
     return foo( 'bar' );
-} );
+};
         ]]>
         </code>
         <code title="Invalid: Using create_function().">
         <![CDATA[
-add_action( 'init', <em>create_function( '',
+$function = <em>create_function</em>(
+    '',
     'return foo( "bar" );'
-)</em> );
+);
         ]]>
         </code>
     </code_comparison>