remount: Add checkpoint instructions with warning

When debugging an issue before boot complete, it may be necessary to
remount before checkpointing can finish. This gives instructions on
ending checkpointing early, allowing a remount, along with a warning to
not do that unless you know what you are doing.

Bug: 252989722
Test: Displays text in log
Change-Id: Ibdd39464b980ec531a900e162469a884f9096904