cmd/internal/obj/arm64: add NOOP description in Go assembly syntax

This Patch describes NOOP in Go assembly syntax and gives Go assembly
example and corresponding GNU assembly example.

Change-Id: I9db659cc5e3dc6b1f1450f2064255af8872d4b1c
Reviewed-on: https://go-review.googlesource.com/c/go/+/207400
Run-TryBot: eric fang <eric.fang@arm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/internal/obj/arm64/doc.go

@@ -53,12 +53,17 @@ Special Cases.
 
 (3) No need to add "W" suffix: LDARB, LDARH, LDAXRB, LDAXRH, LDTRH, LDXRB, LDXRH.
 
+(4) In Go assembly syntax, NOP is a zero-width pseudo-instruction serves generic purpose, nothing
+related to real ARM64 instruction. NOOP serves for the hardware nop instruction. NOOP is an alias of
+HINT $0.
+
   Examples:
     VMOV V13.B[1], R20      <=>      mov x20, v13.b[1]
     VMOV V13.H[1], R20      <=>      mov w20, v13.h[1]
     JMP (R3)                <=>      br x3
     CALL (R17)              <=>      blr x17
     LDAXRB (R19), R16       <=>      ldaxrb w16, [x19]
+    NOOP                    <=>      nop
 
 
 Register mapping rules