document StackEntry

NLLs have since removed its use, but polonius still currently makes use
of it.

compiler/rustc_borrowck/src/dataflow.rs

@@ -239,6 +239,10 @@ pub fn calculate_borrows_out_of_scope_at_location<'tcx>(
     prec.borrows_out_of_scope_at_location
 }
 
+/// An entry in the CFG walk stack of the scope computation, recording visited statements within a
+/// block, in case of cycles requiring revisiting the same block but at earlier points.
+// FIXME: `precompute_borrows_out_of_scope` has stopped using this structure, and a similar
+// reorganizing of `precompute_loans_out_of_scope` could also avoid it.
 struct StackEntry {
     bb: mir::BasicBlock,
     lo: usize,