mention behavior of Array#join for nested arrays [ci skip]

The current documentation for Array#join does not mention the
special treatment of nested arrays.

It says:
> Returns a string created by converting each element of the
> array to a string, separated by the given separator.

Expected behavior according to the docs would be:

    [ "a", [1, 2, [:x, :y]], "b" ].join("-")  #=> "a-[1, 2, [:x, :y]]-b"
    # because of:
    [1, 2, [:x, :y]].to_s  #=> "[1, 2, [:x, :y]]"

Actual behavior:

    [ "a", [1, 2, [:x, :y]], "b" ].join("-")  #=> "a-1-2-x-y-b"

because join is applied recursively for nested arrays.

The patch clarifies this behavior.

(Also: small markup and grammar fix.)

Patch by Marcus Stollsteimer <sto.mar@web.de>

[ruby-talk:437238] [ruby-core:79079] [Bug #13130]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57329 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

array.c

@@ -2075,11 +2075,16 @@ 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 $, are nil, it uses empty string.
+ *  If the +separator+ is +nil+, it uses current <code>$,</code>.
+ *  If both the +separator+ and <code>$,</code> are +nil+,
+ *  it uses an empty string.
  *
  *     [ "a", "b", "c" ].join        #=> "abc"
  *     [ "a", "b", "c" ].join("-")   #=> "a-b-c"
+ *
+ *  For nested arrays, join is applied recursively:
+ *
+ *     [ "a", [1, 2, [:x, :y]], "b" ].join("-")   #=> "a-1-2-x-y-b"
  */
 
 static VALUE