Help #29999, or its tests cannot pass.
Also, add some comments to clarify the usage of `TxContext`.
I don't check all usages of `TxContext` because there are too many
(almost 140+). It's a better idea to replace them with `WithTx` instead
of checking them one by one. However, that may be another refactoring
PR.
(cherry picked from commit c6c4d66004c70b24abc8048b39b660b8361a0395)