Clarify line about RLS being unable to handle rustc
Clarify line about RLS being unable to handle rustc

I based this clarification on this [answer](https://www.reddit.com/r/rust/comments/a8mk33/question_about_a_line_from_the_rust_compiler_book/ecbx1ma/) to
my question on Reddit.

rust-lang/rustc-dev-guide,rust-lang/rustc-dev-guide,rust-lang/rustc-dev-guide,rust-lang/rustc-dev-guide