docs: explain FastMCP request IDs for cancellation #1153

Job	Run time
checks / readme-snippets	12s
checks / pre-commit	30s
checks / test (3.10, lowest-direct, ubuntu-latest)	57s
checks / test (3.12, locked, ubuntu-latest)	55s
checks / test (3.11, locked, windows-latest)	2m 48s
checks / test (3.11, lowest-direct, windows-latest)	2m 38s
checks / test (3.12, lowest-direct, windows-latest)	4m 5s
checks / test (3.11, locked, ubuntu-latest)	52s
checks / test (3.10, locked, ubuntu-latest)	57s
checks / test (3.13, locked, ubuntu-latest)	48s
checks / test (3.10, locked, windows-latest)	2m 14s
checks / test (3.11, lowest-direct, ubuntu-latest)	56s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 1s
checks / test (3.12, locked, windows-latest)	3m 15s
checks / test (3.10, lowest-direct, windows-latest)	2m 43s
checks / test (3.14, locked, ubuntu-latest)	2m 22s
checks / test (3.13, lowest-direct, windows-latest)	2m 36s
checks / test (3.14, locked, windows-latest)	3m 57s
checks / test (3.13, locked, windows-latest)	2m 12s
checks / test (3.14, lowest-direct, windows-latest)	2m 19s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 21s
checks / test (3.13, lowest-direct, ubuntu-latest)	51s
all-green	4s
	40m 33s

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