dataman: extend module documentation

src/modules/dataman/dataman.cpp

@@ -1470,6 +1470,15 @@ Multiple backends are supported:
 It is used to store structured data of different types: mission waypoints, mission state and geofence polygons.
 Each type has a specific type and a fixed maximum amount of storage items, so that fast random access is possible.
 
+### Implementation
+Reading and writing a single item is always atomic. If multiple items need to be read/modified atomically, there is
+an additional lock per item type via `dm_lock`.
+
+**DM_KEY_FENCE_POINTS** and **DM_KEY_SAFE_POINTS** items: the first data element is a `mission_stats_entry_s` struct,
+which stores the number of items for these types. These items are always updated atomically in one transaction (from
+the mavlink mission manager). During that time, navigator will try to acquire the geofence item lock, fail, and will not
+check for geofence violations.
+
 )DESCR_STR");
 
 	PRINT_MODULE_USAGE_NAME("dataman", "system");