doc: mention `descriptorprocesspsbt` in psbt.md #252

Job	Run time
macOS 14 native, arm64, no depends, sqlite only, gui	19m 50s
ASan + LSan + UBSan + integer, no depends, USDT	1h 6m 57s
test each commit	0s
Win64 native, VS 2022	2h 25m 7s
	3h 51m 54s

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