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

BPF_ADD   0x00   dst += src

BPF_SUB   0x10   dst -= src

BPF_MUL   0x20   dst \*= src

BPF_DIV   0x30   dst /= src

BPF_DIV   0x30   dst = (src != 0) ? (dst / src) : 0

BPF_OR    0x40   dst \|= src

BPF_AND   0x50   dst &= src

BPF_LSH   0x60   dst <<= src

BPF_RSH   0x70   dst >>= src

BPF_NEG   0x80   dst = ~src

BPF_MOD   0x90   dst %= src

BPF_MOD   0x90   dst = (src != 0) ? (dst % src) : dst

BPF_XOR   0xa0   dst ^= src

BPF_MOV   0xb0   dst = src

BPF_ARSH  0xc0   sign extending shift right

BPF_END   0xd0   byte swap operations (see `Byte swap instructions`_ below)

========  =====  ==========================================================

Underflow and overflow are allowed during arithmetic operations, meaning

the 64-bit or 32-bit value will wrap. If eBPF program execution would

result in division by zero, the destination register is instead set to zero.

If execution would result in modulo by zero, for ``BPF_ALU64`` the value of

the destination register is unchanged whereas for ``BPF_ALU`` the upper

32 bits of the destination register are zeroed.

``BPF_ADD | BPF_X | BPF_ALU`` means::

  dst_reg = (u32) dst_reg + (u32) src_reg;

  dst_reg = dst_reg ^ imm32

Also note that the division and modulo operations are unsigned. Thus, for

``BPF_ALU``, 'imm' is first interpreted as an unsigned 32-bit value, whereas

for ``BPF_ALU64``, 'imm' is first sign extended to 64 bits and the result

interpreted as an unsigned 64-bit value. There are no instructions for

signed division or modulo.

Byte swap instructions

~~~~~~~~~~~~~~~~~~~~~~