Add performance note for parsing source geometry

Adds performance note for parsing source geometry.

(cherry picked from commit 976f5338c5)

doc/classes/NavigationMeshGenerator.xml

@@ -47,6 +47,7 @@
 			<description>
 				Parses the [SceneTree] for source geometry according to the properties of [param navigation_mesh]. Updates the provided [param source_geometry_data] resource with the resulting data. The resource can then be used to bake a navigation mesh with [method bake_from_source_geometry_data]. After the process is finished the optional [param callback] will be called.
 				[b]Note:[/b] This function needs to run on the main thread or with a deferred call as the SceneTree is not thread-safe.
+				[b]Performance:[/b] While convenient, reading data arrays from [Mesh] resources can affect the frame rate negatively. The data needs to be received from the GPU, stalling the [RenderingServer] in the process. For performance prefer the use of e.g. collision shapes or creating the data arrays entirely in code.
 			</description>
 		</method>
 	</methods>

doc/classes/NavigationServer3D.xml

@@ -685,6 +685,7 @@
 			<description>
 				Parses the [SceneTree] for source geometry according to the properties of [param navigation_mesh]. Updates the provided [param source_geometry_data] resource with the resulting data. The resource can then be used to bake a navigation mesh with [method bake_from_source_geometry_data]. After the process is finished the optional [param callback] will be called.
 				[b]Note:[/b] This function needs to run on the main thread or with a deferred call as the SceneTree is not thread-safe.
+				[b]Performance:[/b] While convenient, reading data arrays from [Mesh] resources can affect the frame rate negatively. The data needs to be received from the GPU, stalling the [RenderingServer] in the process. For performance prefer the use of e.g. collision shapes or creating the data arrays entirely in code.
 			</description>
 		</method>
 		<method name="query_path" qualifiers="const">