Document the behavior of the --force-non-staged flag.

This flag is helpful for teams developing binaries packaged under APEX
files, since it allows a quicker dev cycle by installing new apexes
without rebooting. The fact that it's undocumented makes it harder to
know it exists.

Test: flashed and then 'adb shell pm'
Change-Id: I451389555fa91b170b167d177a35a4dc34c25dc9