When the BUG() function was added in d8193743e0 (usage.c: add BUG()
function, 2017-05-12) these docs added in 1f23cfe0ef (doc: document
error handling functions and conventions, 2014-12-03) were not
updated. Let's do that.
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>