RWLock: Add deadlock example
RWLock: Add deadlock example

Suggested in https://github.com/rust-lang/rust/pull/82596 but it was
a bit too late.

Signed-off-by: Miguel Ojeda <b1a5a4e53a08f19a6a1ffdf06efc0a3c0c374186@kernel.org>

graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust