Auto merge of #8766 - weihanglo:cmd/tree-all-targets, r=ehuss
Auto merge of #8766 - weihanglo:cmd/tree-all-targets, r=ehuss

cargo-tree: mention special target `all` in CLI help text

Fixes #8567

Actually, `cargo help tree` has already got a [description about `--target=all`](https://github.com/rust-lang/cargo/blob/3045228ee139f970ccc892aa5c34c0f3cb70ee06/src/doc/man/cargo-tree.md#tree-options) in tree options:

```console
$ cargo help tree
...
OPTIONS
   Tree Options
        ...
       --target triple
           Filter dependencies matching the given target-triple. The default is the host platform. Use the value all to include all targets.
...

tshepang/cargo,eminence/cargo,nipunn1313/cargo,rustyhorde/cargo,tshepang/cargo,nipunn1313/cargo,rustyhorde/cargo,eminence/cargo,eminence/cargo,nipunn1313/cargo,rust-lang/cargo,rust-lang/cargo,eminence/cargo,tshepang/cargo,rustyhorde/cargo,rust-lang/cargo,rustyhorde/cargo,rust-lang/cargo,tshepang/cargo,nipunn1313/cargo