Warn the user some points might be lost when significant location change monitoring is not available

[Frizlab]

Significant location change monitoring allows the app to be relaunched if it was killed by the system in the background when a significant location change is detected, which in turns allows us to resume recording the location and relaunch the location tracking.
If location change monitoring is not available, a recording might stop at a given point when the system kills the app, and never be resumed if the user does not relaunch the app.