docs: accurately document ZstdDecompressor.decompress()
docs: accurately document ZstdDecompressor.decompress()

Its behavior around multiple frames and extra input was under-specified.
Let's explicitly call out the currently implemented behavior.

Related to #59 and #181 and probably other issues.

indygreg/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard