Document why Decoder::new might return Err

[edmorley]

Migrating from flate2 to the zstd crate, it was surprising to me to find that zstd::stream::read::Decoder::new returns an io::Result:

zstd-rs/src/stream/read/mod.rs

Lines 24 to 31 in e470f00

	impl<R: Read> Decoder<'static, BufReader<R>> {
	/// Creates a new decoder.
	pub fn new(reader: R) -> io::Result<Self> {
	let buffer_size = zstd_safe::DCtx::in_size();
	Self::with_buffer(BufReader::with_capacity(buffer_size, reader))
	}
	}

Compare this to flate2::read::GzDecoder::new, which returns the decoder directly:
https://docs.rs/flate2/latest/flate2/read/struct.GzDecoder.html#method.new

I was hoping the rustdocs for zstd::stream::read::Decoder::new might explain why it is fallible, so I could determine whether specific user facing error handling was required for this failure mode, or whether it was unlikely to occur in practice.

From inspection, it seems the only two fallible calls during the creation of the decoder are here:

zstd-rs/src/stream/raw.rs

Lines 147 to 150 in e470f00

	context.init().map_err(map_error_code)?;
	context
	.load_dictionary(dictionary)
	.map_err(map_error_code)?;

...which are:

1. Initialising the decompression context using zstd_sys::ZSTD_initDStream
2. Loading the dictionary using zstd_sys::ZSTD_DCtx_loadDictionary

Checking the zstd source for ZSTD_initDStream I see:
https://github.com/facebook/zstd/blob/ff7a151f2e6c009b657d9f798c2d9962b0e3feb5/lib/decompress/zstd_decompress.c#L1747-L1754

...where the code comment suggests it can't fail?

Similarly, checking ZSTD_DCtx_loadDictionary I see the fallible paths appear to only be encountered if trying to set the dictionary when the context is in the wrong state (which can't happen unless zstd-safe has a bug), or when a non-empty dictionary is provided (and Decoder::new sets an empty dictionary):
https://github.com/facebook/zstd/blob/ff7a151f2e6c009b657d9f798c2d9962b0e3feb5/lib/decompress/zstd_decompress.c#L1697-L1711

...so unless I'm missing something it appears Decoder::new() can never fail in practice?