zstd_safe::decompress: it is not documented, that you need to have enough capacity

[safinaskar]

I tried to pass a vector as a first argument to zstd_safe::decompress. And it turned out I should have enough capacity to store all decompressed data. Please, document this

[gyscos]

Indeed, as documented it's a very thin wrapper around ZSTD_decompress, and all the same caveats and requirements apply. I added a paragraph to describe this particular point.