Make rust-docs package optional, not installed by default

[brson]

Today the rust-docs package is a required component. Now that rustup supports component installation it can be made optional. This change needs to be done in rust-buildbot.

[durka]

I think it should still be default. Nobody wants to discover at the airport that they should have included just one more rustup incantation.

[nagisa]

Very duplicate-ish of #24117

[nagisa]

@brson why exactly changing defaults in rustup is not enough? Doesn’t it already download the separate packages? It seems to me like it should be as easy as flipping a boolean somewhere in rustup.

[Mark-Simulacrum]

Making the docs component optional, but still installed by default seems like the way to go forward here; that should avoid the missing docs at airport problem, but still allow those who don't want docs to not have to download them.

[japaric]

Rust documentation is now optional and opt-in when installing via rustup

[alexcrichton]

This was done, and then undone, so I'm going to close.