Fixes AsyncIO performance regression for small files

[yoniko]

• Do not use threaded AsyncIO when handling small files.
• Some typo / doc fixes.
• Benchmark:

binary				corpus	real_time	user_time	sys_time
zstd-1.5.2		silesia	0.620000	0.462000	0.150000
zstd-1.5.4-no-asyncio	silesia	0.622000	0.464000	0.148000
zstd-1.5.4-asyncio	silesia	0.520000	0.496000	0.158000
zstd-1.5.2		enwik8	0.342000	0.274000	0.060000
zstd-1.5.4-no-asyncio	enwik8	0.342000	0.272000	0.066000
zstd-1.5.4-asyncio	enwik8	0.290000	0.270000	0.086000
zstd-1.5.2		github	1.528000	0.254000	1.260000
zstd-1.5.4-no-asyncio	github	1.820000	0.258000	1.520000
zstd-1.5.4-asyncio	github	1.812000	0.270000	1.528000