Document NavigationServer API navigation agent callbacks

Document NavigationServer API navigation agent callbacks.

(cherry picked from commit 10c400ca82431bc57dd05152b648e4a096b566cd)

doc/classes/Navigation2DServer.xml

@@ -44,7 +44,8 @@
 			<argument index="2" name="method" type="String" />
 			<argument index="3" name="userdata" type="Variant" default="null" />
 			<description>
-				Callback called at the end of the RVO process.
+				Callback called at the end of the RVO process. If a callback is created manually and the agent is placed on a navigation map it will calculate avoidance for the agent and dispatch the calculated [code]safe_velocity[/code] to the [code]receiver[/code] object with a signal to the chosen [code]method[/code] name.
+				[b]Note:[/b] Created callbacks are always processed independently of the SceneTree state as long as the agent is on a navigation map and not freed. To disable the dispatch of a callback from an agent use [method agent_set_callback] again with a [code]null[/code] object as the [code]receiver[/code].
 			</description>
 		</method>
 		<method name="agent_set_map" qualifiers="const">

doc/classes/NavigationServer.xml

@@ -43,7 +43,8 @@
 			<argument index="2" name="method" type="String" />
 			<argument index="3" name="userdata" type="Variant" default="null" />
 			<description>
-				Callback called at the end of the RVO process.
+				Callback called at the end of the RVO process. If a callback is created manually and the agent is placed on a navigation map it will calculate avoidance for the agent and dispatch the calculated [code]safe_velocity[/code] to the [code]receiver[/code] object with a signal to the chosen [code]method[/code] name.
+				[b]Note:[/b] Created callbacks are always processed independently of the SceneTree state as long as the agent is on a navigation map and not freed. To disable the dispatch of a callback from an agent use [method agent_set_callback] again with a [code]null[/code] object as the [code]receiver[/code].
 			</description>
 		</method>
 		<method name="agent_set_map" qualifiers="const">