Document RichTextLabel not supporting entangled BBCode tags

See #44634.

(cherry picked from commit 3b6612e48edfdc6356095353ed3c2d38ad97b214)

doc/classes/RichTextLabel.xml

@@ -6,6 +6,7 @@
 	<description>
 		Rich text can contain custom text, fonts, images and some basic formatting. The label manages these as an internal tag stack. It also adapts itself to given width/heights.
 		[b]Note:[/b] Assignments to [member bbcode_text] clear the tag stack and reconstruct it from the property's contents. Any edits made to [member bbcode_text] will erase previous edits made from other manual sources such as [method append_bbcode] and the [code]push_*[/code] / [method pop] methods.
+		[b]Note:[/b] RichTextLabel doesn't support entangled BBCode tags. For example, instead of using [code][b]bold[i]bold italic[/b]italic[/i][/code], use [code][b]bold[i]bold italic[/i][/b][i]italic[/i][/code].
 		[b]Note:[/b] Unlike [Label], RichTextLabel doesn't have a [i]property[/i] to horizontally align text to the center. Instead, enable [member bbcode_enabled] and surround the text in a [code][center][/code] tag as follows: [code][center]Example[/center][/code]. There is currently no built-in way to vertically align text either, but this can be emulated by relying on anchors/containers and the [member fit_content_height] property.
 	</description>
 	<tutorials>