doc: self-protection: Add information about STACKLEAK feature

Memory poisoning

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When releasing memory, it is best to poison the contents (clear stack on

syscall return, wipe heap memory on a free), to avoid reuse attacks that

rely on the old contents of memory. This frustrates many uninitialized

variable attacks, stack content exposures, heap content exposures, and

use-after-free attacks.

When releasing memory, it is best to poison the contents, to avoid reuse

attacks that rely on the old contents of memory. E.g., clear stack on a

syscall return (``CONFIG_GCC_PLUGIN_STACKLEAK``), wipe heap memory on a

free. This frustrates many uninitialized variable attacks, stack content

exposures, heap content exposures, and use-after-free attacks.

Destination tracking

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