[TextServer] Add notes about high level text/font classes and getting active server instance.

doc/classes/TextServer.xml

@@ -5,6 +5,16 @@
 	</brief_description>
 	<description>
 		[TextServer] is the API backend for managing fonts and rendering text.
+		[b]Note:[/b] This is a low-level API, consider using [TextLine], [TextParagraph], and [Font] classes instead.
+		This is an abstract class, so to get the currently active [TextServer] instance, use the following code:
+		[codeblocks]
+		[gdscript]
+		var ts = TextServerManager.get_primary_interface()
+		[/gdscript]
+		[csharp]
+		var ts = TextServerManager.GetPrimaryInterface();
+		[/csharp]
+		[/codeblocks]
 	</description>
 	<tutorials>
 	</tutorials>