add comments to mention sort.reverse!

For array.c (Array#sort) and enum.c (Enumerable#sort_by),
add comments mentioning that sort.reverse! / sort_by { ... }.reverse!
can/should be used to reverse the result. [ci skip]

array.c

@@ -2819,6 +2819,11 @@ rb_ary_sort_bang(VALUE ary)
  *     ary.sort                     #=> ["a", "b", "c", "d", "e"]
  *     ary.sort {|a, b| b <=> a}    #=> ["e", "d", "c", "b", "a"]
  *
+ *  To produce the reverse order, the following can also be used
+ *  (and may be faster):
+ *
+ *    ary.sort.reverse!             #=> ["e", "d", "c", "b", "a"]
+ *
  *  See also Enumerable#sort_by.
  */
 

enum.c

@@ -1220,6 +1220,10 @@ sort_by_cmp(const void *ap, const void *bp, void *data)
  *
  *     sorted = Dir["*"].sort_by { |f| test(?M, f) }
  *     sorted   #=> ["mon", "tues", "wed", "thurs"]
+ *
+ *  To produce the reverse of a specific order, the following can  be used:
+ *
+ *    ary.sort_by { ... }.reverse!
  */
 
 static VALUE