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.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@36421 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

array.c

@@ -1808,6 +1808,8 @@ rb_ary_join(VALUE ary, VALUE sep)
  *
  *  Returns a string created by converting each element of the array to
  *  a string, separated by the given +separator+.
+ *  If the +separator+ is +nil+, it uses current $,.
+ *  If both the +separator+ and $, are nil, it uses empty string.
  *
  *     [ "a", "b", "c" ].join        #=> "abc"
  *     [ "a", "b", "c" ].join("-")   #=> "a-b-c"