Warn users of Tier 2 platforms, and explain what that means

TL;DR: Please notify users of Tier 2 platforms a) that it is a Tier 2 platform, and b) what that means.

Describe the problem you are trying to solve

• I have an AArch64 MacBook.
• I tried to use rustup doc, which failed because the files were not there.
• I tried to use rustup component add rust-docs, which failed because toolchain 'stable-aarch64-apple-darwin' does not contain component 'rust-docs' for target 'aarch64-apple-darwin'; did you mean 'rustc-docs'?.
• After much searching, I found a forum post saying that "Tier 2 platforms do not build documentation".
• I followed the link and confirmed that AArch64 macOS is indeed Tier 2. However, that link does not reiterate under the Tier 2 heading that documentation is not available.

Describe the solution you'd like

1. An explicit note on the Platform Support page that Tier 2 does not get documentation.

2. Some kind of notice to the user to explain what's going on, for example:

   % rustup doc
   error: couldn't open browser: command 'open' did not execute successfully; exit status: 1
   The file /Users/eth/.rustup/toolchains/stable-aarch64-apple-darwin/share/doc/rust/html/index.html does not exist.
   
   Have you installed the 'rust-docs' component?
   

   or

   % rustup component add rust-docs
   error: toolchain 'stable-aarch64-apple-darwin' does not contain component 'rust-docs' for target 'aarch64-apple-darwin'; did you mean 'rustc-docs'?
   
   note: 'aarch64-apple-darwin' is a Tier 2 platform, and so the 'rust-docs' component is not available. Please see https://doc.rust-lang.org/rustc/platform-support.html for more information.