docs: autoupdater's fallback option (#786)

docs/features/autoupdater.rst

@@ -70,5 +70,11 @@ These commands can be used on a node:
 
    # Force update check, even when the updater is disabled
    autoupdater -f
+   
+::
+
+   # If fallback is true the updater will perform an update only if 
+   # the timespan given by the priority and another 24h have passed
+   autoupdater --fallback