Add note on attempts of using `Self` as a normal generic arg

[hkmatsumoto]

Given the following code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=53b02445623677643befa3e7dbe0f3b4

fn foo<Self>() {}

The current output is:

error: expected identifier, found keyword `Self`
 --> src/lib.rs:1:8
  |
1 | fn foo<Self>() {}
  |        ^^^^ expected identifier, found keyword

The diagnostic itself is right but confusing; it lacks why Self is a reserved keyword (IIUC it's because Self is special-cased to represent traits or ADTs in its implementations).

[hkmatsumoto]

@rustbot claim