Clarify keepalive interval

1. On https://github.com/moodleuulm/moodle-local_session_keepalive#1-general-functionality, a note should be added that the keepalive interval has to be smaller (but not much smaller) than the Moodle session lifetime.
2. Optionally, it would be nice to have a check and warning message on settings.php if the proportion of both values does not make sense.