File Tree Diff: small documentation to mention the file selector (#12011)
Briefly explanation about how to use visual doc diff.
Closes #12009
<!-- readthedocs-preview docs start -->
---
:books: Documentation previews :books:
- User's documentation (`docs`):
https://docs--12011.org.readthedocs.build/en/12011/
<!-- readthedocs-preview docs end -->
<!-- readthedocs-preview dev start -->
- Developer's documentation (`dev`):
https://dev--12011.org.readthedocs.build/en/12011/
<!-- readthedocs-preview dev end -->
---------
Co-authored-by: Eric Holscher <25510+ericholscher@users.noreply.github.com>