docs: add SET_DISPLAY_TEXT documentation

Signed-off-by:  Eric Callahan <arksine.code@gmail.com>

docs/Command_Templates.md

@@ -138,8 +138,13 @@ This is quite useful if you want to change the behavior of certain commands like
 [gcode_macro M117]
 rename_existing: M117.1
 gcode:
-  M117.1 { rawparams }
-  M118 { rawparams }
+  {% if rawparams %}
+    {% set escaped_msg = rawparams|replace('"', '\\"') %}
+    SET_DISPLAY_TEXT MSG="{escaped_msg}"
+    RESPOND TYPE=command MSG="{escaped_msg}"
+  {% else %}
+    SET_DISPLAY_TEXT
+  {% endif %}
 ```
 
 ### The "printer" Variable

docs/G-Codes.md

@@ -295,6 +295,11 @@ provides the following standard G-Code commands:
 - Display Message: `M117 <message>`
 - Set build percentage: `M73 P<percent>`
 
+Also provided is the following extended G-Code command:
+- `SET_DISPLAY_TEXT MSG=<message>`: Performs the equivalent of M117,
+  setting the supplied `MSG` as the current display message.  If
+  `MSG` is omitted the display will be cleared.
+
 ### [dual_carriage]
 
 The following command is available when the