Document `RichTextLabel.append_bbcode()` not closing previously open tags

This closes #42216.

(cherry picked from commit 3e1457f51747975599c07c8b6aa25435f12cfa22)

doc/classes/RichTextLabel.xml

@@ -42,6 +42,7 @@
 			</argument>
 			<description>
 				Parses [code]bbcode[/code] and adds tags to the tag stack as needed. Returns the result of the parsing, [constant OK] if successful.
+				[b]Note:[/b] Using this method, you can't close a tag that was opened in a previous [method append_bbcode] call. This is done to improve performance, especially when updating large RichTextLabels since rebuilding the whole BBCode every time would be slower. If you absolutely need to close a tag in a future method call, append the [member bbcode_text] instead of using [method append_bbcode].
 			</description>
 		</method>
 		<method name="clear">
@@ -287,7 +288,7 @@
 		</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.
+			[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.
 		</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.