Mention that bbcode_text += string is a bad idea

doc/classes/RichTextLabel.xml

@@ -278,6 +278,7 @@
 		</member>
 		<member name="bbcode_text" type="String" setter="set_bbcode" getter="get_bbcode" default="&quot;&quot;">
 			The label's text in BBCode format. Is not representative of manual modifications to the internal tag stack. Erases changes made by other methods when edited.
+			[b]Note:[/b] It is unadvised to use [code]+=[/code] operator with [code]bbcode_text[/code] (e.g. [code]bbcode_text += "some string"[/code]) as it replaces the whole text and can cause slowdowns. Use [method append_bbcode] for adding text instead.
 		</member>
 		<member name="custom_effects" type="Array" setter="set_effects" getter="get_effects" default="[  ]">
 		</member>