Clarify set intersection and union documentation

* array.c (rb_ary_and): clarify that set intersection returns the
  unique elements common to both arrays.

* array.c (rb_ary_or): clarify that union preserves the order from
  the given arrays.

- Most know what intersection means, but saying the operation
  excludes duplicates could be misleading ([1] & [1], duplicates
  excluded, might mean a result of []).
- Instead, saying intersection returns the unique elements common to both
  arrays is more concise and less ambiguous.
- The set union's documentation was incomplete in its describing
  preservation of order. Saying union preserves the order of the
  original array neglects the idea that the order of the elements
  in both arrays, as given, will be preserved.
- Instead, saying set union preserves the order from the given arrays (and
  adding an example) fully demonstrates the idea.

[Fix GH-1273] [ci skip]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@53958 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ChangeLog

@@ -1,3 +1,11 @@
+Sun Feb 28 10:19:47 2016  Ryan T. Hosford  <tad.hosford@gmail.com>
+
+	* array.c (rb_ary_and): clarify that set intersection returns the
+	  unique elements common to both arrays.
+
+	* array.c (rb_ary_or): clarify that union preserves the order from
+	  the given arrays.
+
 Sat Feb 27 17:05:29 2016  Martin Duerst  <duerst@it.aoyama.ac.jp>
 
 	* enc/unicode/case-folding.rb, casefold.h: Reducing size of TitleCase

array.c

@@ -4098,13 +4098,12 @@ rb_ary_diff(VALUE ary1, VALUE ary2)
  *  call-seq:
  *     ary & other_ary      -> new_ary
  *
- *  Set Intersection --- Returns a new array containing elements common to the
- *  two arrays, excluding any duplicates. The order is preserved from the
- *  original array.
+ *  Set Intersection --- Returns a new array containing unique elements common to the
+ *  two arrays. The order is preserved from the original array.
  *
  *  It compares elements using their #hash and #eql? methods for efficiency.
  *
- *     [ 1, 1, 3, 5 ] & [ 1, 2, 3 ]                 #=> [ 1, 3 ]
+ *     [ 1, 1, 3, 5 ] & [ 3, 2, 1 ]                 #=> [ 1, 3 ]
  *     [ 'a', 'b', 'b', 'z' ] & [ 'a', 'b', 'c' ]   #=> [ 'a', 'b' ]
  *
  *  See also Array#uniq.
@@ -4150,11 +4149,12 @@ ary_hash_orset(st_data_t *key, st_data_t *value, st_data_t arg, int existing)
  *     ary | other_ary     -> new_ary
  *
  *  Set Union --- Returns a new array by joining +ary+ with +other_ary+,
- *  excluding any duplicates and preserving the order from the original array.
+ *  excluding any duplicates and preserving the order from the given arrays.
  *
  *  It compares elements using their #hash and #eql? methods for efficiency.
  *
  *     [ "a", "b", "c" ] | [ "c", "d", "a" ]    #=> [ "a", "b", "c", "d" ]
+ *     [ "c", "d", "a" ] | [ "a", "b", "c" ]    #=> [ "c", "d", "a", "b" ]
  *
  *  See also Array#uniq.
  */