Mention Multimesh custom data will be compressed to half in Compatibi…

…lity

doc/classes/MultiMesh.xml

@@ -65,7 +65,7 @@
 			<param index="0" name="instance" type="int" />
 			<param index="1" name="custom_data" type="Color" />
 			<description>
-				Sets custom data for a specific instance. [param custom_data] is a [Color] type only to contain 4 floating-point numbers.
+				Sets custom data for a specific instance. [param custom_data] is a [Color] type only to contain 4 floating-point numbers. Note each number is stored in 32 bits in the Forward+ and Mobile rendering methods, but is packed into 16 bits in Compatibility.
 				For the custom data to be used, ensure that [member use_custom_data] is [code]true[/code].
 				This custom instance data has to be manually accessed in your custom shader using [code]INSTANCE_CUSTOM[/code].
 			</description>