Add common commands section in help text

[dingxiangfei2009]

Judging from user feedback coming out of Debian distros, it appears that after installing rustup from the package manager it is not immediately clear which command actually installs the toolchain. It is true that rustup from rustup.rs one-liner makes one-stop installation, Debian packaging actually skips the essential rustup-init step as of writing for a good reason. This left a few Debian users, who I talked to recently, feel lost in their first attempt to use the toolchain.

This patch adds a small text so that after installation of rustup, which normally does not invoke the toolchain installation for the user, the user can still receive guidance on how to set up the local toolchain immediately.

[djc]

I think it would be good to try and address this problem, but not sure adding a "common commands" section to the end is the best way to do it. Wonder if we can add this to the top, either as part of Usage: or even before that.

[rami3l]

I think it would be good to try and address this problem, but not sure adding a "common commands" section to the end is the best way to do it.

I would like to disagree with @djc on that matter, as when a command is executed, the first thing I see is the last lines of its output, which is different from when reading a normal doc page.

---

OTOH I do think the case for Debian is still somewhat particular, since installing via APT technically still has quite limited support notably because it explicitly bypasses the rustup-init stage.

We have tried to improve the situation in both our docs and, in my case, its homebrew packaging:

> brew install rustup
[..]
==> Caveats
To initialize `rustup`, set a default toolchain:
  rustup default stable

If you have `rust` installed, ensure you have "$(brew --prefix rustup)/bin"
before "$(brew --prefix)/bin" in your $PATH:
  https://rust-lang.github.io/rustup/installation/already-installed-rust.html

So the policy that I had in the past would be to acknowledge this as the package manager/maintainer's responsibility. Can something similar be done on Debian's side?

[dingxiangfei2009]

I would also prefer to leave the common commands as the last lines of the help text, so that this information does not scroll away in the terminal print out.

As for Debian, I am currently researching a way to update the package description of rustup so that one can be guided to initiate the missing init step, like how homebrew guides the users.

[rami3l]

LGTM modulo one tiny bit :)