Make it easier to not download docs at all.

[brson]

The 'rust' package includes docs, which take up a bunch of room, slowing the download. Some people don't want docs, and downloading is a waste.

Problem here is that it's now impossible for tools like rustup/multirust to determine which version of cargo to pair with each revision of rust. Secondary problem is that the archive date for rustc/cargo packages may be different from the archive date of the rust package, so one may not get the toolchain they were expecting.

One possible (probably easiest) solution is to add an 'online' installer, that doesn't actually include the bins, but knows where the components live. This can be added to rust-installer.

cc brson/multirust#33

[nagisa]

gzipped documentation takes up only about 7MB of the package. With xz (see #21724) it is an essentially free 3MB. Might not be worth having separate doc-less artefacts.

[steveklabnik]

Triage: no change.

Personal note: as @nagisa says, it's very small, and offline docs are quite useful. Lots of people ask about "how do i install the docs locally" on IRC, I would prefer to just keep them.

[nagisa]

I think this is essentially resolved. We distribute rustc, rust-std and rust-docs packages, which in most cases should be preferred to the one-big-bundle packages anyway.

[brson]

With the rustup component subcommand we can make the docs an optional package in the manifests.

[japaric]

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

[alexcrichton]

Closing due to @japaric's last comment