In addition to `foo:1.2.3`, we now support `foo@1.2.3` for pkgids.  We
are also making it the default way of rendering pkgid's for the user.

With cargo-add in rust-lang#10472, we've decided to only use `@` in it and to add
it as an alternative to `:` in the rest of cargo.  `cargo-add`
originally used `@`.  When preparing it for merge, I switched to `:` to
be consistent with pkgids. When discussing this, it was felt `@` has
precedence in too many tools to switch to `:` but that we should instead
switch pkgid's to use `@`, in a backwards compatible way.

See also https://internals.rust-lang.org/t/feedback-on-cargo-add-before-its-merged/16024/26?u=epage

Since the `cargo pkgid` docs were so brief, I figured I'd focus on the
expected common case and only cover `@` while the reference mentions
both in the grammar.