docs(tvix/eval) comments for various fields

Change-Id: I8dcddf2b419761e475e71215c199eef2f7dc61dc
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7028
Autosubmit: Adam Joseph <adam@westernsemico.com>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/eval/src/value/mod.rs

@@ -43,8 +43,13 @@ pub enum Value {
     // Internal values that, while they technically exist at runtime,
     // are never returned to or created directly by users.
     Thunk(Thunk),
+
+    // See [`compiler::compile_select_or()`] for explanation
     AttrNotFound,
+
+    // this can only occur in Chunk::Constants and nowhere else
     Blueprint(Rc<Lambda>),
+
     DeferredUpvalue(StackIdx),
     UnresolvedPath(PathBuf),
 }