Revise wording of `maybe-uninit.intro`

The proposed wording here was taken from the documentation in the
standard library.  That text is a bit loose.  It says that the type is
"used to construct uninitialized values of type `T`", but of course
we're using it to construct *initialized* values of type `T`, not
*uninitialized* ones.  And that's only one use case.

It's indeed a bit tricky to concisely describe `MaybeUninit<T>`.
Let's try to do a bit better.