eBPF.md: Document atomic exchange and atomic compare-and-write

Atomic exchange and atomic compare-and-write are selected when the
immediate value for an atomic operation is set to BPF_XCHG (0xe0) |
BPF_FETCH (1) and BPF_CMPXCHG (0xf0) | BPF_FETCH (1), respectively.

This is supported in the kernel since commit:
5ffa25502b5a ("bpf: Add instructions for atomic_[cmp]xchg")

Author: qmonnet

eBPF.md

@@ -234,10 +234,14 @@ Opcode             | Mnemonic                   | Pseudocode
 -------------------|----------------------------|---------------------------------------------------------
 0xdb (imm == 0x00) | add [dst+off], src         | (dst + off) += src
 0xdb (imm == 0x01) | fetch_add [dst+off], src   | src = dst, (dst + off) += src
+0xdb (imm == 0xe1) | xchg [dst+off], src        | src = (dst + off), (dst + off) = src
+0xdb (imm == 0xf1) | cmpxchg [dst+off], src     | r0 = (dst + off), (dst + off) = src if (dst + off) == r0
 
 ### 32-bit
 
 Opcode             | Mnemonic                   | Pseudocode (uint32_t * casts omitted for readability)
 -------|----------------------------|---------------------------------------------------------------------
 0xc3 (imm == 0x00) | add32 [dst+off], src       | (dst + off) += src
 0xc3 (imm == 0x01) | fetch_add32 [dst+off], src | src = dst, (dst + off) += src
+0xc3 (imm == 0xe1) | xchg32 [dst+off], src      | src = (dst + off), (dst + off) = src
+0xc3 (imm == 0xf1) | cmpxchg32 [dst+off], src   | r0 = (dst + off), (dst + off) = src if (dst + off) == r0