feat(cli/help): discuss rustup toolchain install

[rami3l]

Addresses #4513 (comment).

[djc]

This is great! Should we also show this for update?

Tiny nit: I would use lowercase after ":".

[rami3l]

@djc Thanks for the review! I've addressed the concerns accordingly along with a tiny fix that turns out to be a leftover from #3596.

[rami3l]

@epage Remaining concern from #4551: do you think it's a good idea to indent all discussions by 2 spaces instead of 4, or are you sure that clap segments are indented by 2 spaces? The current style (see the svg diffs in this PR) is a bit confusing...

[epage]

Clap indents sections by 2 spaces but for flags, that two spaces is for where the short flags is placed.

https://github.com/clap-rs/clap/blob/c7c761f988684ad97c8b2c521b05cf7f8192b3eb/clap_builder/src/output/mod.rs#L21

https://github.com/clap-rs/clap/blob/c7c761f988684ad97c8b2c521b05cf7f8192b3eb/clap_builder/src/output/help_template.rs#L518-L520

https://github.com/clap-rs/clap/blob/c7c761f988684ad97c8b2c521b05cf7f8192b3eb/clap_builder/src/output/help_template.rs#L1071-L1072

[epage]

Note that that we reduced the padding between clap v3 and v4, see https://github.com/clap-rs/clap/blob/master/CHANGELOG.md#400---2022-09-28

[rami3l]

Note that that we reduced the padding between clap v3 and v4, see https://github.com/clap-rs/clap/blob/master/CHANGELOG.md#400---2022-09-28

@epage Ah I see why the current indentation is what it is. Maybe that deserves another PR to fix? For this patch in particular let's just stay with the old style then.