Mention that modifying text erases BBCode stacks

(cherry picked from commit 3f0e9da5a457409b3223fe8ec77af590fd7a6baf)

doc/classes/RichTextLabel.xml

@@ -243,7 +243,7 @@
 		</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 the [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, unless you absolutely need to close a tag that was opened in an earlier method call.
+			[b]Note:[/b] It is unadvised to use the [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. It will also erase all BBCode that was added to stack using [code]push_*[/code] methods. Use [method append_bbcode] for adding text instead, unless you absolutely need to close a tag that was opened in an earlier method call.
 		</member>
 		<member name="custom_effects" type="Array" setter="set_effects" getter="get_effects" default="[  ]">
 			The currently installed custom effects. This is an array of [RichTextEffect]s.