Docs / ParameterValues/RemovedPCREModifiers: various tweaks

* Add proper XML header.
    Note: this is something which only recently became an option as there previously was no schema available.
* Style: have `CDATA` tags on their own lines.
    This makes the XML code style consistent with currently existing XML docs.
* Expand the issue description a little.
* Use `Cross-version compatible` and `PHP </> #.#` style prefixes in the code samples.
    This improves consistency with the other XML docs and makes the code samples more descriptive in the context of PHPCompatibility.
* Add `<em>` tags to the code samples to allow for highlighting the specific issue the sniff is looking for.

PHPCompatibility/Docs/ParameterValues/RemovedPCREModifiersStandard.xml

@@ -1,23 +1,34 @@
-<documentation title="Removed PCRE Modifiers">
-    <standard><![CDATA[
-The /e modifier is deprecated since PHP 5.5 and removed since PHP 7.0. Use preg_replace_callback or preg_replace_array_callback instead.
-    ]]></standard>
+<?xml version="1.0"?>
+<documentation xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xsi:noNamespaceSchemaLocation="https://phpcsstandards.github.io/PHPCSDevTools/phpcsdocs.xsd"
+    title="Removed PCRE Modifiers"
+    >
+    <standard>
+    <![CDATA[
+    The /e modifier, as could be used in regexes passed to the PCRE `preg_replace()` and `preg_filter()` functions, is deprecated since PHP 5.5 and removed since PHP 7.0.
+    Use `preg_replace_callback()` or `preg_replace_array_callback()` instead.
+    ]]>
+    </standard>
     <code_comparison>
-        <code title="Valid"><![CDATA[
-preg_replace_callback(
+        <code title="Cross-version compatible: using preg_replace_callback()">
+        <![CDATA[
+<em>preg_replace_callback</em>(
     '/^(.?)/',
-    function ($matches) {
+    <em>function ($matches) {
         return strtoupper($matches[1]);
-    },
+    }</em>,
     $input
 );
-        ]]></code>
-        <code title="Invalid"><![CDATA[
+        ]]>
+        </code>
+        <code title="PHP &lt; 7.0: using the /e modifier">
+        <![CDATA[
 preg_replace(
-    '/^(.?)/e',
-    "strtoupper('\\1')",
+    <em>'/^(.?)/e'</em>,
+    <em>"strtoupper('\\1')"</em>,
     $input
 );
-        ]]></code>
+        ]]>
+        </code>
     </code_comparison>
 </documentation>