Bug 1866262 - [a11y] Clarify comment for accessibility string.

Previously the comment that explained the a11y_heading string was not
very clear and it didn't explain with what the "%s" will be replaced.

app/src/main/res/values/strings.xml

@@ -2289,6 +2289,6 @@
     <string name="a11y_action_label_read_article">read the article</string>
     <!-- Action label for links to the Firefox Pocket website. Talkback will append this to say "Double tap to open link to learn more". -->
     <string name="a11y_action_label_pocket_learn_more">open link to learn more</string>
-    <!-- Talkback will announce the heading element as "Heading". -->
+    <!-- Content description for headings announced by accessibility service. The first parameter is the text of the heading. Talkback will announce the first parameter and then speak the word "Heading" indicating to the user that this text is a heading for a section. -->
     <string name="a11y_heading">%s, Heading</string>
 </resources>