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/function.rs

@@ -9,10 +9,20 @@ use crate::{
     upvalues::{UpvalueCarrier, Upvalues},
 };
 
+/// The opcodes for a thunk or closure, plus the number of
+/// non-executable opcodes which are allowed after an OpClosure or
+/// OpThunk referencing it.  At runtime `Lambda` is usually wrapped
+/// in `Rc` to avoid copying the `Chunk` it holds (which can be
+/// quite large).
 #[derive(Debug, PartialEq)]
 pub struct Lambda {
     // name: Option<NixString>,
     pub(crate) chunk: Chunk,
+
+    /// Number of upvalues which the code in this Lambda closes
+    /// over, and which need to be initialised at
+    /// runtime.  Information about the variables is emitted using
+    /// data-carrying opcodes (see [`OpCode::DataLocalIdx`]).
     pub(crate) upvalue_count: usize,
 }