docs: explain FastMCP request IDs for cancellation #1146

Job	Run time
checks / readme-snippets	11s
checks / pre-commit	38s
checks / test (3.10, locked, ubuntu-latest)	50s
checks / test (3.11, lowest-direct, ubuntu-latest)	52s
checks / test (3.10, locked, windows-latest)	2m 13s
checks / test (3.10, lowest-direct, ubuntu-latest)	58s
checks / test (3.11, lowest-direct, windows-latest)	2m 48s
checks / test (3.10, lowest-direct, windows-latest)	2m 29s
checks / test (3.12, lowest-direct, windows-latest)	3m 36s
checks / test (3.12, lowest-direct, ubuntu-latest)	58s
checks / test (3.12, locked, ubuntu-latest)	52s
checks / test (3.13, locked, ubuntu-latest)	48s
checks / test (3.13, lowest-direct, ubuntu-latest)	50s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 18s
checks / test (3.13, lowest-direct, windows-latest)	2m 36s
checks / test (3.11, locked, windows-latest)	2m 32s
checks / test (3.14, locked, windows-latest)	3m 25s
checks / test (3.12, locked, windows-latest)	3m 26s
checks / test (3.11, locked, ubuntu-latest)	49s
checks / test (3.14, lowest-direct, windows-latest)	2m 36s
checks / test (3.13, locked, windows-latest)	2m 54s
checks / test (3.14, locked, ubuntu-latest)	2m 18s
all-green	2s
	39m 59s

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