Closes #4537 - Adds description for onboarding placeholder string (#4542)

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

@@ -646,7 +646,8 @@
     The first parameter is the name of the app (e.g. Firefox Preview) -->
     <string name="onboarding_firefox_account_header">Get the most out of %s.</string>
     <!-- text for the firefox account onboarding card header when we detect you're already signed in to
-        another Firefox browser. (The word `Firefox` should not be translated -->
+        another Firefox browser. (The word `Firefox` should not be translated)
+        The first parameter is the email of the detected user's account -->
     <string name="onboarding_firefox_account_auto_signin_header_2">You are signed in as %s on another Firefox browser on this phone. Would you like to sign in with this account?</string>
     <!-- text for the button to confirm automatic sign-in -->
     <string name="onboarding_firefox_account_auto_signin_confirm">Yes, sign me in</string>