[fenix] For https://github.com/mozilla-mobile/fenix/pull/21392 - Update new strings descriptions.

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

@@ -1933,7 +1933,7 @@
     <string name="pocket_stories_placeholder_text">Discover more</string>
     <!-- Title of an app feature. Smaller than a heading.-->
     <string name="pocket_stories_feature_title">Powered by Pocket</string>
-    <!-- Caption which concatenates the text of "pocket_stories_feature_learn_more"  -->
+    <!-- Caption for describing a certain feature. The placeholder is for a clickable text (eg: Learn more) which will load an url in a new tab when clicked.  -->
     <string name="pocket_stories_feature_caption">Part of the Firefox family. %s</string>
     <!-- Clickable text for opening an external link for more information about Pocket. -->
     <string name="pocket_stories_feature_learn_more">Learn more</string>