cmd/compile: document go:noinline compiler directive

[hiltjo]

What version of Go are you using (go version)?

go-git (commit a2bd5c5) and 1.7.3

Issue

I noticed the go:noinline compiler directive is not documented on the page
https://golang.org/cmd/compile/ .

Fix

A patch is attached to fix it. The wording can probably be expanded and improved though.

https://cow.codemadness.org/go/patches/0001-cmd-compile-document-go-noinline-compiler-directive.patch

Edit: attaching patches on github sucks, so I made a link to the patch.

[0xmohit]

Please refer to Contributing Guidelines.

[ianlancetaylor]

The go:noinline comment is only intended for testing the compiler. It is not intended for general use. Therefore, it is not documented in the user documentation. See the discussion at #12312.

(It would be nice if there were better internal documentation for the compiler, which is where this should be documented, but that is a separate issue.)