grammar : add comments to new grammar file

grammars/json_arr.gbnf

@@ -1,6 +1,15 @@
+# This is the same as json.gbnf but we restrict whitespaces at the end of the root array
+# Useful for generating JSON arrays
+
 root   ::= arr
 value  ::= object | array | string | number | ("true" | "false" | "null") ws
 
+arr  ::=
+  "{\n\t[\n" ws (
+            value
+    (",\n" ws value)*
+  )? "\t]\n}"
+
 object ::=
   "{" ws (
             string ":" ws value
@@ -13,12 +22,6 @@ array  ::=
     ("," ws value)*
   )? "]" ws
 
-arr  ::=
-  "[\n" ws (
-            value
-    (",\n" ws value)*
-  )? "]"
-
 string ::=
   "\"" (
     [^"\\] |