Make Dispatch::Queue#apply documentation less misleading

Claiming that the blocks are executed in parallel is not correct
since it depends on the type of queue being used, and even in that
case concurrency is not the same things as parallelism.

In the example, the array grows dynamically and the assignment
being performed in the block would not be thread-safe
(concurrency-safe?).

gcd.c

@@ -612,12 +612,13 @@ rb_block_applier(void *data, size_t ii)
  *  call-seq:
  *    gcdq.apply(count) { |index| block }
  *
- *  Runs a block count number of times in parallel via 
+ *  Runs a block _count_ number of times asynchronously via
  *  dispatch_apply(3)[http://developer.apple.com/mac/library/DOCUMENTATION/Darwin/Reference/ManPages/man3/dispatch_apply.3.html],
- *  passing in an index and waiting until all of them are done
+ *  passing in an index and waiting until all of them are done.
+ *  You must use a concurrent queue to run the blocks concurrently.
  *  
  *     gcdq = Dispatch::Queue.new('doc')
- *     @result = []
+ *     @result = Array.new(5)
  *     gcdq.apply(5) {|i| @result[i] = i*i }
  *     p @result  #=> [0, 1, 4, 9, 16]
  *