docs: update metadata documentation

Document the "gcode_start_byte" and "gcode_end_byte" fields.

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

docs/api_changes.md

@@ -1,6 +1,15 @@
 This document keeps a record of all changes to Moonraker's remote
 facing APIs.
 
+### November 12th 2020
+- Two new fields have been added to the gcode metadata:
+  - `gcode_start_byte`:  Indicates the byte position in the
+    file where the first "Gxx" or "Mxx" command is detected.
+  - `gcode_end_byte`:  Indicates the byte position in the
+    file where the last "Gxx" or "Mxx" command is detected.
+  These fields may be used to more accurately predict print
+  progress based on the file size.
+
 ### November 11th 2020
 - The `server.websocket.id` API has been added.  This returns a
   unique ID that Moonraker uses to track each client connection.

docs/web_api.md

@@ -489,6 +489,8 @@ path relative to the specified "root".  Note that if the query st
     object_height: <mm>,
     estimated_time: <time_in_seconds>,
     filament_total: <mm>,
+    gcode_start_byte: <byte_location_of_first_gcode_command>,
+    gcode_end_byte: <byte_location_of_last_gcode_command>,
     thumbnails: [
       {
         width: <in_pixels>,