docs: deprecated.rst: Add uninitialized_var()

Nothing should be using this macro, and the entire idea of tricking the
compiler into silencing such warnings is a mistake.

Cc: Jonathan Corbet <[e-mail]>
Cc: "Gustavo A. R. Silva" <[e-mail]>
Cc: Joe Perches <[e-mail]>
Cc: [e-mail]
Reviewed-by: Nick Desaulniers <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>

Documentation/process/deprecated.rst

···
 "unreachable" conditions. (For example, see commits like `this one
 <https://git.kernel.org/linus/d4689846881d160a4d12a514e991a740bcb5d65a>`_.)
 
+uninitialized_var()
+-------------------
+For any compiler warnings about uninitialized variables, just add
+an initializer. Using the uninitialized_var() macro (or similar
+warning-silencing tricks) is dangerous as it papers over `real bugs
+<https://lore.kernel.org/lkml/[e-mail]/>`_
+(or can in the future), and suppresses unrelated compiler warnings
+(e.g. "unused variable"). If the compiler thinks it is uninitialized,
+either simply initialize the variable or make compiler changes. Keep in
+mind that in most cases, if an initialization is obviously redundant,
+the compiler's dead-store elimination pass will make sure there are no
+needless variable writes.
+
+As Linus has said, this macro
+`must <https://lore.kernel.org/lkml/CA+55aFw+Vbj0i=[e-mail]/>`_
+`be <https://lore.kernel.org/lkml/[e-mail]/>`_
+`removed <https://lore.kernel.org/lkml/CA+55aFz2500WfbKXAx8s67wrm9=[e-mail]/>`_.
+
 open-coded arithmetic in allocator arguments
 --------------------------------------------
 Dynamic size calculations (especially multiplication) should not be