DOC: Added that threads can now contain a metadata stream #4882

Job	Run time
pytest on windows	7m 8s
Check code style issues	42s
pytest on 3.7 (crypto-lib: cryptography)	5m 3s
Build & verify package	15s
pytest on 3.8 (crypto-lib: cryptography)	5m 43s
pytest on 3.9 (crypto-lib: cryptography)	5m 23s
pytest on 3.10 (crypto-lib: cryptography)	5m 49s
pytest on 3.11 (crypto-lib: cryptography)	5m 6s
pytest on 3.12 (crypto-lib: cryptography)	6m 25s
pytest on 3.7 (crypto-lib: pycryptodome)	4m 50s
pytest on 3.7 (crypto-lib: none)	4m 34s
Combine & check coverage.	22s
	51m 20s

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