code: public static boolean isText(ByteBuf buf, Charset charset) {
          return isText(buf, buf.readerIndex(), buf.readableBytes(), charset);
      }
docs: Returns {@code true} if the given {@link ByteBuf} is valid text using the given {@link Charset},
  otherwise return {@code false}.
  
  @param buf The given {@link ByteBuf}.
  @param charset The specified {@link Charset}.