Adding TOOLCHAIN_HELP to rustup help install

[peschkaj]

Adds the help from TOOLCHAIN_HELP to rustup help install. There are likely other places we can add this and get additional help about <toolchain> into rustup.

This should fix #483

[peschkaj]

@brson Does this work for you?

[brson]

OK, sorry for the delay.

I'd prefer to see the help for update and install be similar, with an INSTALL_HELP text that is substantially similar to UPDATE_HELP. If <toolchain> isn't sufficiently explained then it needs to be explained in both places.

Instead of explaining in full what a toolchain is perhaps these two could contain more abbreviated text with pointers to more information:

'toolchain' specifies a toolchain name, such as 'stable', 'nightly', or '1.8.0'. For more information see rustup help toolchain.

[peschkaj]

👍 will revise.

Thanks for the eyeballs.

[brson]

Thanks!