Document beginless, endless ranges in Range class documentation

range.c

@@ -2135,6 +2135,15 @@ range_count(int argc, VALUE *argv, VALUE range)
  *   end
  *   a # => [2, 4, 6, 8, 10]
  *
+ * A range can be both beginless and endless.  For literal beginless, endless
+ * ranges, at least the beginning or end of the range must be given as an
+ * explicit nil value. It is recommended to use an explicit nil beginning and
+ * implicit nil end, since that is what Ruby uses for Range#inspect:
+ *
+ *   (nil..)    # => (nil..)
+ *   (..nil)    # => (nil..)
+ *   (nil..nil) # => (nil..)
+ *
  * == Ranges and Other Classes
  *
  * An object may be put into a range if its class implements