Correct help on how bg-dexopt-job interacts with the job scheduler.

The intentional and documented behaviour from commit cb5f41ea was that a
job started through `cmd package bg-dexopt-job` would run concurrently
with one started by the job scheduler. However this changed in commit
f992cb31 when cancellation was added and runIdleOptimizationsNow got
removed.

Also document that bg-dexopt-job takes an optional list of packages.

#codehealth

Test: n/a
Bug: 179094324
Bug: 156537504
Change-Id: I53db26c9244b240fe2b198b396c5a2fbd9d16ada