docs(tvix/eval): upvalues.rs: define "upvalue", comment with_stack r/5157

This commit adds a comment for the benefit of non-Lua-users
explaining what an upvalue is.  It also adds a comment explaining
what `with_stack` does, including a brief reminder of nix's
slightly-unusual-but-well-justified handling of this construct.

Signed-off-by: Adam Joseph <adam@westernsemico.com>
Change-Id: If6d0e133292451af906e1c728e34010f99be8c7c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7007
Reviewed-by: j4m3s <james.landrein@gmail.com>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

0 files changed, 0 insertions, 0 deletions