Add documentation for -Zverbose-asm

src/doc/unstable-book/src/compiler-flags/verbose-asm.md

@@ -0,0 +1,70 @@
+# `verbose-asm`
+
+The tracking issue for this feature is: [#126802](https://github.com/rust-lang/rust/issues/126802).
+
+------------------------
+
+This enables passing `-Zverbose-asm` to get contextual comments added by LLVM.
+
+Sample code:
+
+```rust
+#[no_mangle]
+pub fn foo(a: i32, b: i32) -> i32 {
+    a + b
+}
+```
+
+Default output:
+
+```asm
+foo:
+        push    rax
+        add     edi, esi
+        mov     dword ptr [rsp + 4], edi
+        seto    al
+        jo      .LBB0_2
+        mov     eax, dword ptr [rsp + 4]
+        pop     rcx
+        ret
+.LBB0_2:
+        lea     rdi, [rip + .L__unnamed_1]
+        mov     rax, qword ptr [rip + core::panicking::panic_const::panic_const_add_overflow::h9c85248fe0d735b2@GOTPCREL]
+        call    rax
+
+.L__unnamed_2:
+        .ascii  "/app/example.rs"
+
+.L__unnamed_1:
+        .quad   .L__unnamed_2
+        .asciz  "\017\000\000\000\000\000\000\000\004\000\000\000\005\000\000"
+```
+
+With `-Zverbose-asm`:
+
+```asm
+foo:                                    # @foo
+# %bb.0:
+        push    rax
+        add     edi, esi
+        mov     dword ptr [rsp + 4], edi        # 4-byte Spill
+        seto    al
+        jo      .LBB0_2
+# %bb.1:
+        mov     eax, dword ptr [rsp + 4]        # 4-byte Reload
+        pop     rcx
+        ret
+.LBB0_2:
+        lea     rdi, [rip + .L__unnamed_1]
+        mov     rax, qword ptr [rip + core::panicking::panic_const::panic_const_add_overflow::h9c85248fe0d735b2@GOTPCREL]
+        call    rax
+                                        # -- End function
+.L__unnamed_2:
+        .ascii  "/app/example.rs"
+
+.L__unnamed_1:
+        .quad   .L__unnamed_2
+        .asciz  "\017\000\000\000\000\000\000\000\004\000\000\000\005\000\000"
+
+                                        # DW_AT_external
+```