io: remove UTF8 specified document for RuneReader

io: remove UTF8 specified document for RuneReader

RuneReader is fine with UTF16 or any other character encoding

Fixes #49178

Change-Id: I08a5ac205e095349d451d3b60411eaeebc3aa563
Reviewed-on: https://go-review.googlesource.com/c/go/+/359334
Trust: Meng Zhuo <mzh@golangcn.org>
Reviewed-by: Meng Zhuo <mzh@golangcn.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Meng Zhuo <mzh@golangcn.org>
TryBot-Result: Go Bot <gobot@golang.org>