rustup help install doesn't say what <toolchain> is

[kornelski]

rustup help install

Update Rust toolchains

USAGE:
install [FLAGS] <toolchain>

FLAGS:
-h, --help Prints help information

ARGS:
<toolchain>

This is very terse and doesn't tell me what values are valid for <toolchain> (related: #482). The error message isn't helping either:

rustup install foo
error: toolchain 'foo' is not installed