Move debuginfo docs from doc.rs module to doc.md file

And use `#[doc = include_str!("doc.md")]` in `mod.rs` so the docs are
rendered as if they were inline in the root module.

compiler/rustc_codegen_llvm/src/debuginfo/mod.rs

@@ -1,5 +1,4 @@
-// See doc.rs for documentation.
-mod doc;
+#![doc = include_str!("doc.md")]
 
 use rustc_codegen_ssa::mir::debuginfo::VariableKind::*;