Improve label for multiple additional css classes (#15634)

* Improve label for multiple additional css classes

Add clarity around additional css classes so users know that multiple
classes are supported and the proper separator between classes.

* Add "(es)" so new label is "Additional CSS Class(es)"

* Add help attribute with help message: "Space separate multiple classes."

* Update copy per review

packages/block-editor/src/hooks/custom-class-name.js

@@ -60,13 +60,14 @@ export const withInspectorControl = createHigherOrderComponent( ( BlockEdit ) =>
 					<BlockEdit { ...props } />
 					<InspectorAdvancedControls>
 						<TextControl
-							label={ __( 'Additional CSS Class' ) }
+							label={ __( 'Additional CSS Class(es)' ) }
 							value={ props.attributes.className || '' }
 							onChange={ ( nextValue ) => {
 								props.setAttributes( {
 									className: nextValue !== '' ? nextValue : undefined,
 								} );
 							} }
+							help={ __( 'Separate multiple classes with spaces.' ) }
 						/>
 					</InspectorAdvancedControls>
 				</>