docs: Clarified behavior of legacy apps running on M

Per bug, emphasized that if an app targets SDK 22 or lower, it uses
the install-time permissions model even if it's running on M.
However, also added the note that an M user can *revoke* an app's
permissions after install, which can cause unpredictable behavior.

See first comment for doc stage location.

bug: 25470298
Change-Id: Ia05a1f784595e75bf7b6835dbfdeffcd458f0022