doc: explain why we substract 4000 from `DEFAULT_BLOCK_MAX_WEIGHT` #370

Job	Run time
macOS 14 native, arm64, no depends, sqlite only, gui	22m 51s
ASan + LSan + UBSan + integer, no depends, USDT	1h 9m 14s
Win64 native, VS 2022	2h 23m 20s
test each commit	0s
	3h 55m 25s

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