docs: deprecated.rst: Add BUG()-family

Linus continues to remind[1] people to stop using the BUG()-family of
functions. We should have this better documented (even if checkpatch.pl
has been warning[2] since 2015), so add more details to deprecated.rst,
as a distinct place to point people to for guidance.

[1] https://lore.kernel.org/lkml/CAHk-=whDHsbK3HTOpTF=ue_o04onRwTEaK_ZoJp_fjbqq4+=Jw@mail.gmail.com/
[2] https://git.kernel.org/linus/9d3e3c705eb395528fd8f17208c87581b134da48



Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/202003141524.59C619B51A@keescook


Signed-off-by: Jonathan Corbet <corbet@lwn.net>