Clarify help for update

[johnthagen]

It's mentioned in the docs, but when you run rustup --help:

...
update         Update Rust toolchains
...

rustup update also updates rustup itself. While I think this is a good idea, I was a little surprised by the action. rustup update --help does explains things well.

Could consider changing the top level help to something like:

update         Update Rust toolchains and rustup

Since you can also do rustup self update, maybe this could be confusing though.

[brson]

Hm, this is a little tricky because while rustup update updates rustup itself, rustup update nightly does not, so documenting the subcommand as "Update Rust toolchains and rustup" also seems a bit confusing.

Still, I think your suggestion is an improvement. Let's do it.

[johnthagen]

@brson PR created.