Document trailing `self` in paths

The Reference says that `self` can only appear as the first segment of
a path.  Further, the `use` chapter describes `{self}` brace syntax as
the way to bind the parent entity of a `use` path under its own name.

In rust-lang/rust#155137, we're relaxing these restrictions: `self`
may now also appear as the last segment of a path (preceded by `::`)
as long as the preceding path resolves to a module, enumeration,
or trait.  In a `use` path, `use P::self [as name]` is equivalent
to `use P::{self [as name]}`.  In non-`use` paths, forms such as
`type Ty = P::self` and `pub(in P::self)` are now accepted.

Let's update the Reference to reflect this.