bpf, docs: Fix modulo zero, division by zero, overflow, and underflow

Fix modulo zero, division by zero, overflow, and underflow. Also clarify how
a negative immediate value is used in unsigned division.

Signed-off-by: Dave Thaler <dthaler@microsoft.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230124001218.827-1-dthaler1968@googlemail.com