docs: link ConfigAwareSingleFragmentActivity

[david-allison]

The class was not discoverable

@ericli3690 would this have helped in 378a2b4?

Checklist

• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
• UI Changes: You have tested your change using the Google Accessibility Scanner

[ericli3690]

WOW! I did NOT know this activity existed... how did that slip by me 😅
Thanks thanks!! Will integrate into #21140