Surprising behavior of ByteString literals via IsString

At work, we discovered a somewhat surprising behavior of ByteString's `IsString` instance and interaction with `OverloadedStrings`.

The following REPL session demonstrates the issue:

```
λ> BS.unpack $ T.encodeUtf8 ("bla語" :: Text)
[98,108,97,232,170,158]
λ> BS.unpack $ ("bla語" :: BS.ByteString)
[98,108,97,158]
λ> T.decodeUtf8 $ ("bla語" :: BS.ByteString)
*** Exception: Cannot decode byte '\x9e': Data.Text.Internal.Encoding.decodeUtf8: Invalid UTF-8
```

The `IsString` instance calls `packChars` which calls [`c2w`](https://github.com/haskell/bytestring/blob/master/Data/ByteString/Internal.hs#L529-L534), which silently truncates the bytes.

I'd be happy to put together a PR to document the behavior of the `IsString` instance.

I think I expected it to encode the string using the source encoding. I don't know whether or not that's a feasible or desirable change.