[DOC] Fix and improve array slicing example in range.c

* [DOC] Fix typo in range.c

  In the example of the beginless range used for array slicing,
  '..' range literal was used while the '...' literal was expected

* [DOC] Add example for array slicing in range.c

  Add an example for the array slice with the beginless range
  using the '..' range literal

* [DOC] Add comments for array slicing in range.c

  Add comments to make crystal clear what the '..' and '...'
  range literals do when used for array slicing as beginless range

range.c

@@ -2438,8 +2438,12 @@ range_overlap(VALUE range, VALUE other)
  * A beginless range may be used to slice an array:
  *
  *  a = [1, 2, 3, 4]
- *  r = (..2) # => nil...2
- *  a[r]      # => [1, 2]
+ *  # Include the third array element in the slice
+ *  r = (..2)  # => nil..2
+ *  a[r]       # => [1, 2, 3]
+ *  # Exclude the third array element from the slice
+ *  r = (...2) # => nil...2
+ *  a[r]       # => [1, 2]
  *
  * \Method +each+ for a beginless range raises an exception.
  *