PHP/RestrictedPHPFunctions: add XML documentation (#2693)

* Add documentation for WordPress.PHP.RestrictedPHPFunctions

* 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.

* Explain why in the standard description

Following the suggestion in PR 2687, this commit improves the
standard description to explain why these functions must not
be used.

The phrasing is kept generic instead of mentioning
create_function() specifically, since the sniff name suggests
it could be extended with more functions in the future.


Co-authored-by: gogdzl <diego.rodrigues@automattic.com>

Author: rodrigoprimo

WordPress/Docs/PHP/RestrictedPHPFunctionsStandard.xml

@@ -0,0 +1,34 @@
+<?xml version="1.0"?>
+<documentation xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xsi:noNamespaceSchemaLocation="https://phpcsstandards.github.io/PHPCSDevTools/phpcsdocs.xsd"
+    title="Restricted PHP Functions"
+    >
+    <standard>
+    <![CDATA[
+    Certain PHP functions must not be used. Use the recommended alternatives instead.
+    ]]>
+    </standard>
+    <code_comparison>
+        <code title="Valid: Using an anonymous function (closure).">
+        <![CDATA[
+add_action(
+    'init',
+    <em>function () {
+        return foo( 'bar' );
+    }</em>
+);
+        ]]>
+        </code>
+        <code title="Invalid: Using create_function().">
+        <![CDATA[
+add_action(
+    'init',
+    <em>create_function(
+        '',
+        'return foo( "bar" );'
+    )</em>
+);
+        ]]>
+        </code>
+    </code_comparison>
+</documentation>