Add an example to the `CanvasItem.draw_string()` documentation

This closes https://github.com/godotengine/godot-docs/issues/3374.

doc/classes/CanvasItem.xml

@@ -316,7 +316,16 @@
 			<argument index="4" name="clip_w" type="int" default="-1">
 			</argument>
 			<description>
-				Draws a string using a custom font.
+				Draws [code]text[/code] using the specified [code]font[/code] at the [code]position[/code] (top-left corner). The text will have its color multiplied by [code]modulate[/code]. If [code]clip_w[/code] is greater than or equal to 0, the text will be clipped if it exceeds the specified width.
+				[b]Example using the default project font:[/b]
+				[codeblock]
+				# If using this method in a script that redraws constantly, move the
+				# `default_font` declaration to a member variable assigned in `_ready()`
+				# so the Control is only created once.
+				var default_font = Control.new().get_font("font")
+				draw_string(default_font, Vector2(64, 64), "Hello world")
+				[/codeblock]
+				See also [method Font.draw].
 			</description>
 		</method>
 		<method name="draw_style_box">

doc/classes/Font.xml

@@ -26,6 +26,7 @@
 			</argument>
 			<description>
 				Draw [code]string[/code] into a canvas item using the font at a given position, with [code]modulate[/code] color, and optionally clipping the width. [code]position[/code] specifies the baseline, not the top. To draw from the top, [i]ascent[/i] must be added to the Y axis.
+				See also [method CanvasItem.draw_string].
 			</description>
 		</method>
 		<method name="draw_char" qualifiers="const">