docs: clarify Streamable HTTP stateless mode semantics and usage #964

Job	Run time
checks / pre-commit	34s
checks / readme-snippets	11s
checks / test (3.10, locked, windows-latest)	2m 8s
checks / test (3.10, lowest-direct, ubuntu-latest)	56s
checks / test (3.11, lowest-direct, windows-latest)	2m 41s
checks / test (3.10, locked, ubuntu-latest)	55s
checks / test (3.11, lowest-direct, ubuntu-latest)	51s
checks / test (3.12, lowest-direct, windows-latest)	3m 31s
checks / test (3.12, lowest-direct, ubuntu-latest)	57s
checks / test (3.10, lowest-direct, windows-latest)	2m 19s
checks / test (3.11, locked, ubuntu-latest)	46s
checks / test (3.12, locked, ubuntu-latest)	53s
checks / test (3.13, locked, windows-latest)	2m 17s
checks / test (3.12, locked, windows-latest)	3m 48s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 19s
checks / test (3.13, locked, ubuntu-latest)	47s
checks / test (3.14, locked, ubuntu-latest)	2m 21s
checks / test (3.13, lowest-direct, ubuntu-latest)	52s
checks / test (3.14, lowest-direct, windows-latest)	2m 30s
checks / test (3.13, lowest-direct, windows-latest)	2m 32s
checks / test (3.11, locked, windows-latest)	2m 37s
checks / test (3.14, locked, windows-latest)	3m 31s
all-green	3s
	39m 19s

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