Add some basic docs to sym and kw modules

I was looking into improving some Clippy documentation but was missing a
place that explains the `kw` and `sym` modules from rustc.

src/librustc_span/symbol.rs

@@ -1153,12 +1153,20 @@ impl Interner {
 }
 
 // This module has a very short name because it's used a lot.
+/// This module contains all the defined keyword `Symbol`s.
+///
+/// Given that `kw` is imported, use them like `kw::keyword_name`.
+/// For example `kw::Loop` or `kw::Break`.
 pub mod kw {
     use super::Symbol;
     keywords!();
 }
 
 // This module has a very short name because it's used a lot.
+/// This module contains all the defined non-keyword `Symbol`s.
+///
+/// Given that `sym` is imported, use them like `sym::symbol_name`.
+/// For example `sym::rustfmt` or `sym::u8`.
 #[allow(rustc::default_hash_types)]
 pub mod sym {
     use super::Symbol;