Update documentation on gun valid mod locations

doc/JSON_INFO.md

@@ -1617,6 +1617,7 @@ Guns can be defined like this:
 "built_in_mods": ["m203"], //An array of mods that will be integrated in the weapon using the IRREMOVABLE tag.
 "default_mods": ["m203"]   //An array of mods that will be added to a weapon on spawn.
 "barrel_length": "30 mL",  // Amount of volume lost when the barrel is sawn. Approximately 9mL per inch is a decent approximation.
+"valid_mod_locations": [ [ "accessories", 4 ], [ "grip", 1 ] ],  // The valid locations for gunmods and the mount of slots for that location.
 ```
 Alternately, every item (book, tool, armor, even food) can be used as gun if it has gun_data:
 ```json