rustdoc: Remove `*` intra-doc alias for `pointer`
It's not valid Rust code and it can easily be confused with a wildcard
glob pattern or something else. People can always use `pointer` instead,
so it's just removing an alias.
It hasn't hit stable yet (I think it's still on nightly), so it's okay
to remove it. (We can always add it back later if we change our mind
too.)
r? `@jyn514`
cc https://github.com/rust-lang/rust/pull/80885#discussion_r554622737
a7f7d70860