docs: clarify Streamable HTTP stateless mode semantics and usage #1803

Job	Run time
checks / readme-snippets	15s
checks / pre-commit	29s
checks / test (3.11, lowest-direct, ubuntu-latest)	51s
checks / test (3.10, locked, ubuntu-latest)	52s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 5s
checks / test (3.11, locked, ubuntu-latest)	51s
checks / test (3.10, lowest-direct, ubuntu-latest)	52s
checks / test (3.11, lowest-direct, windows-latest)	3m 4s
checks / test (3.11, locked, windows-latest)	2m 35s
checks / test (3.10, lowest-direct, windows-latest)	2m 25s
checks / test (3.14, locked, windows-latest)	3m 7s
checks / test (3.12, locked, ubuntu-latest)	52s
checks / test (3.13, locked, ubuntu-latest)	48s
checks / test (3.13, lowest-direct, windows-latest)	3m 18s
checks / test (3.13, locked, windows-latest)	2m 11s
checks / test (3.10, locked, windows-latest)	2m 19s
checks / test (3.13, lowest-direct, ubuntu-latest)	56s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 21s
checks / test (3.12, locked, windows-latest)	3m 19s
checks / test (3.14, locked, ubuntu-latest)	2m 24s
checks / test (3.14, lowest-direct, windows-latest)	2m 21s
checks / test (3.12, lowest-direct, windows-latest)	3m 40s
all-green	3s
	39m 58s

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