docs: Updated desc. of lockNow() method and its overload

Explains in more complete detail who should call lockNow(), when to call the method,
and what happens when there's no lock set on a device.

Test: make ds-docs -j32

Bug: 28831838
Change-Id: I5acc0cbfa63dffe8676e8b60476b584fd43b5bde
1 file changed