Generic/DisallowShortOpenTag: add comment with more details about a test

Author: rodrigoprimo

src/Standards/Generic/Tests/PHP/DisallowShortOpenTagUnitTest.3.inc

@@ -16,4 +16,5 @@ echo '<?';
 Some content <? echo $var; ?> Some longer content to trigger snippet clipping
 
 // Only recognize closing tag after opener.
+// The test below must be the last test in the file because there must be no PHP close tag after it.
 Some?> content <?