compiletest: Improve no_prefer_dynamic docs
compiletest: Improve no_prefer_dynamic docs

This adds some extra docs for the `no-prefer-dynamic` header.

And also a `s/must_compile_successfully/compile_pass`.

`must_compile_successfully` has been renamed to `compile_pass` at some
point in the past and this comment was still referring to the old name.

aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust