rust: revocable: document why &T is not used in RevocableGuard

When a reference appears in a function argument, the reference is
assumed to be valid for the entire duration of that function call; this
is called a stack protector [1]. Because of that, custom pointer types
whose destructor may invalidate the pointee (i.e. they are more similar
to Box<T> than &T) cannot internally use a reference, and must instead
use a raw pointer.

This issue is something that is often missed during unsafe review. For
examples, see [2] and [3]. To ensure that people don't try to simplify
RevocableGuard by changing the raw pointer to a reference, add a comment
to that effect.

Link: https://perso.crans.org/vanille/treebor/protectors.html [1]
Link: https://users.rust-lang.org/t/unsafe-code-review-semi-owning-weak-rwlock-t-guard/95706 [2]
Link: https://lore.kernel.org/all/[email protected]/ [3]
Signed-off-by: Alice Ryhl <[email protected]>
Reviewed-by: Benno Lossin <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[ Adjusted title. - Miguel ]
Signed-off-by: Miguel Ojeda <[email protected]>

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