Dynamic arrays: Fix some doc errors, and introduce the term 'array reference' to reduce confusion.

Discussion thread: http://forum.dlang.org/post/loekwubzykmysrdzwpak@forum.dlang.org

Changes:
- Where it says 'dynamic array', instead say 'array reference'.
  Rationale:
  1) When an array reference is an lvalue, it behaves like a reference, not a dynamic array.
  2) When you shrink and later regrow an array reference, without using assumeSafeAppend(),
     reallocation occurs. This behavior seems reasonable, but it is not expected behavior
     for a dynamic array.
- Retain the term 'dynamic array' when we're referring to the GC-allocated array
  that these types often refer to. Those act like arrays and are somewhat dynamic.
- Clarify expression.dd - the definition of identity for array references. (Initially I thought it was
  incorrect, but I'd misread. Changing it to clarify.)

In arrays.dd:
- Fix some example code which didn't compile (especially around pointers).
- Reorder - introduce slices early, 'slicing' much later. Move "Array Declarations" later still.
- Add code cases around "s = <foo>" where s is a static array. The current version of arrays.dd
  seems to think this is meaningless, but I found otherwise through experimentation.

Notes (things I didn't do):

- abi.dd: The section named "Reference types" is slightly affected. It could add that array references are
  an exception to what it says there.
- d-array-article.dd: Some of this article is affected, it needs careful editing. Perhaps
  the need for it is eliminated but perhaps not.
- ctod.dd: This makes the claim that "D supports dynamic arrays, which can be easily resized", which
  I feel overstates it. The example given only shows growing, not shrinking, and
  it shows only a single array reference giving the impression of a one-to-one relationship
  where there is actually in general a many-to-one relationship.
- My search/replace would have missed where "dynamic array" has a linebreak between the two words
  in the .dd file. I noticed one case in overview.dd (but for that one, I prefer to leave it saying
  "dynamic array").
- Where I found "static or dynamic array" I wrote "static array or array reference".
  I'm inclined to think we don't need the "static" qualifier anymore, so
  maybe these should now read "array or array reference".