Document stack smashing protection

[themaxdavitt]

Stack smashing protection was added in #84197, but src/doc/rustc/src/exploit-mitigations.md was never updated.

@rustbot label A-docs

[mrcnski]

@rcvalle Have you started on this already? I'm available to take this on. I came across the docs before the PR, and was misled about the current state of this feature.

[rcvalle]

@mrcnski Not yet. Sorry about the confusion caused by it. If you'd have time to and would like to contribute, it'd be much appreciated. Feel free to CC me in the PR and I'd be happy to help reviewing it.