README is inconsistent about default toolchains installed on Windows

[Arnavion]

The README says:

Finally, the host may be specified as a target triple. This is most
useful for installing a 32-bit compiler on a 64-bit platform, or for
installing the [MSVC-based toolchain] on Windows. For example:

$ rustup install stable-x86_64-pc-windows-msvc

For convenience, elements of the target triple that are omitted will be
inferred, so the above could be written:

$ rustup install stable-msvc

but

$ rustup toolchain install stable-msvc

info: syncing channel updates for 'stable-i686-pc-windows-msvc'

(The same happens for nightly-msvc)

---

Edit: This later section in the README justifies installing the 32-bit toolchain by default:

By default rustup on Windows configures Rust to target the 32-bit MSVC
ABI, that is the i686-pc-windows-msvc target triple. More
specifically, the toolchains that rustup chooses to install, unless
told otherwise through the [toolchain specification], will be compiled
to run on a i686-pc-windows-msvc host, and will target that platform
by default. When you write rustup update nightly, rustup interprets
it as rustup update nightly-i686-pc-windows-msvc. You can change this
behavior with rustup set default-host or during installation.

so the first section needs to be fixed.

---

However even this second quote implies that msvc would be installed over gnu

When you write rustup update nightly, rustup interprets
it as rustup update nightly-i686-pc-windows-msvc.

which isn't what actually happens:

$ rustup toolchain install stable

info: syncing channel updates for 'stable-i686-pc-windows-gnu'

$ rustup toolchain install nightly

info: syncing channel updates for 'nightly-i686-pc-windows-gnu'

(I have VS 2013 installed.)

[rbtcollins]

Sorry for taking so long to get to this! I think in todays world we probably want to install x86_64 and msvc by default, and consistently, and have the i686 and gnu variants both require active opt-in. We should check if thats not the case, and if not fix it, and align the docs to that.

[workingjubilee]

@rustbot label: +O-windows