Add documentation about Array#join(nil). [Bug#5915]
We talked about this and it is like following:
- maybe ary.join(nil) should always use empty string as a separator, but it disables to specify $, as a seprator through variables.
- if nil means '', the conversion is to_s. to_s is not accepted.
- nil is a special value, so special behavior is allowed.
- use of $ variables is not recommended, so it is not worth to change current behavior.