GH Actions/basics: add check for valid XML docs + fix two docs

.github/workflows/validate.yml

@@ -72,6 +72,10 @@ jobs:
           diff -B ./src/Standards/Squiz/ruleset.xml <(xmllint --format "./src/Standards/Squiz/ruleset.xml")
           diff -B ./src/Standards/Zend/ruleset.xml <(xmllint --format "./src/Standards/Zend/ruleset.xml")
 
+      # Validate the basic well-formedness of the Documentation XML files.
+      - name: Validate documentation XML
+        run: xmllint --noout ./src/Standards/*/Docs/*/*Standard.xml
+
   yamllint:
     name: 'Lint Yaml'
     # Don't run the cronjob in this workflow on forks.

src/Standards/Generic/Docs/WhiteSpace/HereNowdocIdentifierSpacingStandard.xml

@@ -5,14 +5,14 @@
     ]]>
     </standard>
     <code_comparison>
-        <code title="Valid: No space between the <<< and the identifier string.">
+        <code title="Valid: No space between the &lt;&lt;&lt; and the identifier string.">
         <![CDATA[
 $heredoc = <em><<<EOD</em>
 some text
 EOD;
         ]]>
         </code>
-        <code title="Invalid: Whitespace between the <<< and the identifier string.">
+        <code title="Invalid: Whitespace between the &lt;&lt;&lt; and the identifier string.">
         <![CDATA[
 $heredoc = <em><<<   END</em>
 some text

src/Standards/Generic/Docs/WhiteSpace/SpreadOperatorSpacingAfterStandard.xml

@@ -25,7 +25,7 @@ function bar(<em>... </em>$spread) {
     );
 
     bar(
-        [<em>...  </em>$foo ],<em>.../*comment*/</em>array_values($keyedArray)
+        [<em>... </em>$foo ],<em>.../*@*/</em>array_values($keyed)
     );
 }
         ]]>