DOC: Added that threads can now contain a metadata stream #4903

Job	Run time
Build & verify package	16s
pytest on windows	7m 7s
Check code style issues	41s
pytest on 3.7 (crypto-lib: cryptography)	4m 48s
pytest on 3.8 (crypto-lib: cryptography)	4m 50s
pytest on 3.9 (crypto-lib: cryptography)	5m 5s
pytest on 3.10 (crypto-lib: cryptography)	5m 18s
pytest on 3.11 (crypto-lib: cryptography)	4m 31s
pytest on 3.12 (crypto-lib: cryptography)	6m 10s
pytest on 3.7 (crypto-lib: pycryptodome)	4m 54s
pytest on 3.7 (crypto-lib: none)	5m 2s
Combine & check coverage.	23s
	49m 5s

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