Mention Multimesh custom data will be compressed to half in Compatibility

doc/classes/MultiMesh.xml

@@ -57,6 +57,7 @@
 			<param index="1" name="color" type="Color" />
 			<description>
 				Sets the color of a specific instance by [i]multiplying[/i] the mesh's existing vertex colors. This allows for different color tinting per instance.
+				[b]Note:[/b] Each component is stored in 32 bits in the Forward+ and Mobile rendering methods, but is packed into 16 bits in the Compatibility rendering method.
 				For the color to take effect, ensure that [member use_colors] is [code]true[/code] on the [MultiMesh] and [member BaseMaterial3D.vertex_color_use_as_albedo] is [code]true[/code] on the material. If you intend to set an absolute color instead of tinting, make sure the material's albedo color is set to pure white ([code]Color(1, 1, 1)[/code]).
 			</description>
 		</method>
@@ -66,6 +67,7 @@
 			<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.
+				[b]Note:[/b] Each number is stored in 32 bits in the Forward+ and Mobile rendering methods, but is packed into 16 bits in the Compatibility rendering method.
 				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>