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

fs_mgr/fs_mgr_remount.cpp

@@ -175,6 +175,8 @@ bool VerifyCheckpointing() {
     }
     if (checkpointing) {
         LOG(ERROR) << "Cannot use remount when a checkpoint is in progress.";
+        LOG(ERROR) << "To force end checkpointing, call 'vdc checkpoint commitChanges'";
+        LOG(ERROR) << "Warning: this can lead to data corruption if rolled back.";
         return false;
     }
     return true;