documentation of Connection::recv

https://github.com/cloudflare/quiche/blob/be37a83c1e3d9bf88fddf3615734666312035cac/quiche/src/lib.rs#L1779-L1786

In the doc, it is not clear that whether `recv` always processes the whole `buf` unless error out. Looking at the code, it seems to be true. Should the doc clarify about that?

Start at quiche/src/lib.rs around lines 1779-1786 and compare the Connection::recv documentation with the referenced implementation. Confirm whether recv processes the whole buffer unless it returns an error, then clarify the documentation so that behavior is explicit.

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