Document what we expect from the test.

src/Tests/PhpFilterTest.php

@@ -41,7 +41,7 @@ class PhpFilterTest extends PhpTestBase {
     $this->assertNoText('print "SimpleTest PHP was executed!"', "PHP code isn't displayed.");
     $this->assertText('SimpleTest PHP was executed!', 'PHP code has been evaluated.');
 
-    // Cache test.
+    // Verify that cache is disabled for PHP evaluates.
     $this->assertText('Current state is empty', 'PHP code has been evaluated once.');
     \Drupal::state()->set('php_state_test', 'not empty');
     $this->drupalGet('node/' . $node->id());