docs: Update READ_EXTERNAL_STORAGE explanation

* Mention that T+ apps need to request media-specific permissions,
  instead of READ_EXTERNAL_STORAGE, to access other apps' media files
* Add information about how scoped storage reduces the need for apps
  to declare this permission
* Remove errant note about WRITE_EXTERNAL_STORAGE implicitly granting
  READ_EXTERNAL_STORAGE -- the converse is true

Bug: 249520024
Change-Id: I03c354ab878090c0179382dd77014ba70cd7d17f
Test: built docs using go/abtd