array.c: [DOC] Make it clear that #<< modifies receiver

This patch is sent from @selmertsx (morioka shuhei).

[fix GH-1646]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59070 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

array.c

@@ -910,7 +910,10 @@ ary_take_first_or_last(int argc, const VALUE *argv, VALUE ary, enum ary_take_pos
  *  expression returns the array itself, so several appends
  *  may be chained together.
  *
- *     [ 1, 2 ] << "c" << "d" << [ 3, 4 ]
+ *     a = [ 1, 2 ]
+ *     a << "c" << "d" << [ 3, 4 ]
+ *             #=>  [ 1, 2, "c", "d", [ 3, 4 ] ]
+ *     a
  *             #=>  [ 1, 2, "c", "d", [ 3, 4 ] ]
  *
  */