Clarify that 2D cell sizes are defined in pixels

See https://github.com/godotengine/godot/issues/40577#issuecomment-662329021.

doc/classes/ProjectSettings.xml

@@ -873,7 +873,7 @@
 			Size of the hash table used for the broad-phase 2D hash grid algorithm.
 		</member>
 		<member name="physics/2d/cell_size" type="int" setter="" getter="" default="128">
-			Cell size used for the broad-phase 2D hash grid algorithm.
+			Cell size used for the broad-phase 2D hash grid algorithm (in pixels).
 		</member>
 		<member name="physics/2d/default_angular_damp" type="float" setter="" getter="" default="1.0">
 			The default angular damp in 2D.
@@ -1199,7 +1199,7 @@
 		<member name="rendering/vulkan/staging_buffer/texture_upload_region_size_px" type="int" setter="" getter="" default="64">
 		</member>
 		<member name="world/2d/cell_size" type="int" setter="" getter="" default="100">
-			Cell size used for the 2D hash grid that [VisibilityNotifier2D] uses.
+			Cell size used for the 2D hash grid that [VisibilityNotifier2D] uses (in pixels).
 		</member>
 	</members>
 	<constants>