common: NAN and INT32_MAX for default param values (mavlink#1303)

message_definitions/v1.0/common.xml

@@ -1076,7 +1076,7 @@
     <!--  * the data payload of mavlink commands (as used in the COMMAND_INT and COMMAND_LONG messages) -->
     <!-- ALL the entries in the MAV_CMD enum have a maximum of 7 parameters -->
     <enum name="MAV_CMD">
-      <description>Commands to be executed by the MAV. They can be executed on user request, or as part of a mission script. If the action is used in a mission, the parameter mapping to the waypoint/mission message is as follows: Param 1, Param 2, Param 3, Param 4, X: Param 5, Y:Param 6, Z:Param 7. This command list is similar what ARINC 424 is for commercial aircraft: A data format how to interpret waypoint/mission data. See https://mavlink.io/en/guide/xml_schema.html#MAV_CMD for information about the structure of the MAV_CMD entries</description>
+      <description>Commands to be executed by the MAV. They can be executed on user request, or as part of a mission script. If the action is used in a mission, the parameter mapping to the waypoint/mission message is as follows: Param 1, Param 2, Param 3, Param 4, X: Param 5, Y:Param 6, Z:Param 7. This command list is similar what ARINC 424 is for commercial aircraft: A data format how to interpret waypoint/mission data. NaN and INT32_MAX may be used in float/integer params (respectively) to indicate optional/default values (e.g. to use the component's current yaw or latitude rather than a specific value). See https://mavlink.io/en/guide/xml_schema.html#MAV_CMD for information about the structure of the MAV_CMD entries</description>
       <entry value="16" name="MAV_CMD_NAV_WAYPOINT" hasLocation="true" isDestination="true">
         <description>Navigate to waypoint.</description>
         <param index="1" label="Hold" units="s" minValue="0">Hold time. (ignored by fixed wing, time to stay at waypoint for rotary wing)</param>
@@ -4379,7 +4379,7 @@
     </message>
     <message id="39" name="MISSION_ITEM">
       <description>Message encoding a mission item. This message is emitted to announce
-                the presence of a mission item and to set a mission item on the system. The mission item can be either in x, y, z meters (type: LOCAL) or x:lat, y:lon, z:altitude. Local frame is Z-down, right handed (NED), global frame is Z-up, right handed (ENU). See also https://mavlink.io/en/services/mission.html.</description>
+                the presence of a mission item and to set a mission item on the system. The mission item can be either in x, y, z meters (type: LOCAL) or x:lat, y:lon, z:altitude. Local frame is Z-down, right handed (NED), global frame is Z-up, right handed (ENU). NaN may be used to indicate an optional/default value (e.g. to use the system's current latitude or yaw rather than a specific value). See also https://mavlink.io/en/services/mission.html.</description>
       <field type="uint8_t" name="target_system">System ID</field>
       <field type="uint8_t" name="target_component">Component ID</field>
       <field type="uint16_t" name="seq">Sequence</field>
@@ -4641,7 +4641,7 @@
     </message>
     <message id="73" name="MISSION_ITEM_INT">
       <description>Message encoding a mission item. This message is emitted to announce
-                the presence of a mission item and to set a mission item on the system. The mission item can be either in x, y, z meters (type: LOCAL) or x:lat, y:lon, z:altitude. Local frame is Z-down, right handed (NED), global frame is Z-up, right handed (ENU). See also https://mavlink.io/en/services/mission.html.</description>
+                the presence of a mission item and to set a mission item on the system. The mission item can be either in x, y, z meters (type: LOCAL) or x:lat, y:lon, z:altitude. Local frame is Z-down, right handed (NED), global frame is Z-up, right handed (ENU). NaN or INT32_MAX may be used in float/integer params (respectively) to indicate optional/default values (e.g. to use the component's current latitude, yaw rather than a specific value). See also https://mavlink.io/en/services/mission.html.</description>
       <field type="uint8_t" name="target_system">System ID</field>
       <field type="uint8_t" name="target_component">Component ID</field>
       <field type="uint16_t" name="seq">Waypoint ID (sequence number). Starts at zero. Increases monotonically for each waypoint, no gaps in the sequence (0,1,2,3,4).</field>