Editing SceneTree.get_frame() documentaion

to reflect that it returns physics steps that have been processed,
not frames rendered.

fixes #90539

Add reference to physics_frame signal.

Add reference to Engine.get_process_frames

Update doc/classes/SceneTree.xml

Co-authored-by: A Thousand Ships <[email protected]>

doc/classes/SceneTree.xml

@@ -109,7 +109,7 @@
 		<method name="get_frame" qualifiers="const">
 			<return type="int" />
 			<description>
-				Returns how many frames have been processed, since the application started. This is [i]not[/i] a measurement of elapsed time.
+				Returns how many physics process steps have been processed, since the application started. This is [i]not[/i] a measurement of elapsed time. See also [signal physics_frame]. For the number of frames rendered, see [method Engine.get_process_frames].
 			</description>
 		</method>
 		<method name="get_multiplayer" qualifiers="const">