[Files SWA]: Expands fileManagerPrivate APIs documentation.

Adds a caveat about events in fileManagerPrivate APIs. These are always
broadcasted. As a result all privilege applications can listen to all
events of other privileged applications. Therefore they must take a
precaution to discard events not destined for them. In addition they
must not log data delivered via events.

Bug: b/188508702
Change-Id: Ibbfc6909137267f3dc87515a5f6d723a375abc2b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2960330


Commit-Queue: Bo Majewski <majewski@chromium.org>
Reviewed-by: Mattias Nissler <mnissler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#896456}