Mention how to add margins to Button's icon

(cherry picked from commit 2341c7bbd93e934647e25428b3656e439ade2b99)

doc/classes/Button.xml

@@ -41,6 +41,7 @@
 		</member>
 		<member name="icon" type="Texture" setter="set_button_icon" getter="get_button_icon">
 			Button's icon, if text is present the icon will be placed before the text.
+			To edit margin and spacing of the icon, use [code]hseparation[/code] theme property of [Button] and [code]content_margin_*[/code] properties of the used [StyleBox]es.
 		</member>
 		<member name="text" type="String" setter="set_text" getter="get_text" default="&quot;&quot;">
 			The button's text that will be displayed inside the button's area.