Add a couple of comments

Co-Authored-By: Ralf Jung <post@ralfj.de>

Author: bjorn3

src/tools/miri/src/shims/foreign_items.rs

@@ -51,6 +51,8 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
 
  // Some shims forward to other MIR bodies.
  match link_name.as_str() {
+ // This allocator function has forwarding shims synthesized during normal codegen
+ // (see `allocator_shim_contents`); this is where we emulate that behavior.
  // FIXME should use global_fn_name, but mangle_internal_symbol requires a static str.
  name if name == this.mangle_internal_symbol("__rust_alloc_error_handler") => {
  // Forward to the right symbol that implements this function.
@@ -68,6 +70,8 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
  .expect("missing alloc error handler symbol");
  return interp_ok(Some(handler));
  }
+ // Fall through to the `lookup_exported_symbol` below which should find
+ // a `__rust_alloc_error_handler`.
  }
  _ => {}
  }