doc: self-protection: Add information about STACKLEAK feature

Add information about STACKLEAK feature to the "Memory poisoning"
section of self-protection.rst.

Signed-off-by: Alexander Popov <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>

Documentation/security/self-protection.rst

···
 Memory poisoning
 ----------------
 
-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
 --------------------