cmd/internal/obj/riscv: document compressed instructions

cmd/internal/obj/riscv: document compressed instructions

We update the RISC-V assembler documentation to describe how
the RISC-V compressed instruction set is implemented by the
assembler and how compressed instructions can be disabled.

Change-Id: Ic7b1cb1586e6906af78adb8ff5fa10f5fbfde292
Reviewed-on: https://go-review.googlesource.com/c/go/+/719221
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Joel Sing <joel@sing.id.au>
Auto-Submit: Joel Sing <joel@sing.id.au>
Reviewed-by: Meng Zhuo <mengzhuo1203@gmail.com>