docs: Fixed misleading information about permissions

Doc had said that users have to grant all permissions at install time
(which was misleading--users don't have to grant normal permissions--
and wrong if app and device use runtime permissions).

Changed the text to just say that the user needs to explicitly grant
"these permissions" (the permissions being talked about are all
dangerous), and pointed the reader to the Permissions training class.

I'll build and stage, but since it's a one-sentence fix I'll go ahead
and submit if it looks good.

bug: 25412683
Change-Id: Idc17828f2958d4f8900d29e564c05c54bf3ce753