docs: clarify Streamable HTTP stateless mode semantics and usage #965

Job	Run time
checks / pre-commit	28s
checks / readme-snippets	14s
checks / test (3.10, lowest-direct, windows-latest)	2m 45s
checks / test (3.10, locked, ubuntu-latest)	51s
checks / test (3.11, lowest-direct, windows-latest)	2m 37s
checks / test (3.10, lowest-direct, ubuntu-latest)	58s
checks / test (3.12, locked, windows-latest)	3m 19s
checks / test (3.10, locked, windows-latest)	2m 35s
checks / test (3.12, locked, ubuntu-latest)	56s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 18s
checks / test (3.13, locked, ubuntu-latest)	48s
checks / test (3.11, lowest-direct, ubuntu-latest)	56s
checks / test (3.13, lowest-direct, windows-latest)	2m 32s
checks / test (3.11, locked, ubuntu-latest)	51s
checks / test (3.13, lowest-direct, ubuntu-latest)	56s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 4s
checks / test (3.14, lowest-direct, windows-latest)	2m 16s
checks / test (3.13, locked, windows-latest)	2m 17s
checks / test (3.14, locked, windows-latest)	3m 27s
checks / test (3.11, locked, windows-latest)	2m 15s
checks / test (3.12, lowest-direct, windows-latest)	3m 30s
checks / test (3.14, locked, ubuntu-latest)	2m 20s
all-green	4s
	39m 17s

Provide feedback