document decimal_class on c ext Parser#initialize and top JSON module

ext/json/ext/parser/parser.c

@@ -2812,6 +2812,9 @@ static VALUE convert_encoding(VALUE source)
 *   defaults to false.
 * * *object_class*: Defaults to Hash
 * * *array_class*: Defaults to Array
+# * *decimal_class*: Specifies which class to use instead of the default
+#   (Float) when parsing decimal numbers. This class must accept a single
+#   string argument in its constructor.
 */
 static VALUE cParser_initialize(int argc, VALUE *argv, VALUE self)
 {

lib/json.rb

@@ -187,6 +187,19 @@
 #
 # ---
 #
+# Option +decimal_class+ (\Class) specifies which class to use when parsing
+# decimal numbers. This class must accept a single string argument in its
+# constructor. Defaults to Float. BigDecimal will preserve arbitrary precision.
+#
+# With the default, \Float:
+#   source = '3.141592653589793238462643383279'
+#   ruby = JSON.parse(source) # => 3.141592653589793
+# With \BigDecimal:
+#   ruby = JSON.parse(source, {decimal_class: BigDecimal})
+#   ruby # => 0.3141592653589793238462643383279e1
+#
+# ---
+#
 # Option +create_additions+ (boolean) specifies whether to use \JSON additions in parsing.
 # See {\JSON Additions}[#module-JSON-label-JSON+Additions].
 #