Clarify the relationship between `forget()` and `ManuallyDrop`.

As discussed on reddit, this commit addresses two issues with the
documentation of `mem::forget()`:

* The documentation of `mem::forget()` can confuse the reader because of the
  discrepancy between usage examples that show correct usage and the
  accompanying text which speaks of the possibility of double-free.  The
  text that says "if the panic occurs before `mem::forget` was called"
  refers to a variant of the second example that was never shown, modified
  to use `mem::forget` instead of `ManuallyDrop`.  Ideally the documentation
  should show both variants, so it's clear what it's talking about.

  Also, the double free could be fixed just by placing `mem::forget(v)`
  before the construction of `s`.  Since the lifetimes of `s` and `v`
  wouldn't overlap, there would be no point where panic could cause a double
  free.  This could be mentioned, and contrasted against the more robust fix
  of using `ManuallyDrop`.

* This sentence seems unjustified: "For some types, operations such as
  passing ownership (to a funcion like `mem::forget`) requires them to
  actually be fully owned right now [...]".  Unlike C++, Rust has no move
  constructors, its moves are (possibly elided) bitwise copies.  Even if you
  pass an invalid object to `mem::forget`, no harm should come to pass
  because `mem::forget` consumes the object and exists solely to prevent
  drop, so there no one left to observe the invalid state state.

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