docs: release: Add note on firmware loader mode #1740

Job	Run time
config (RELEASE 2,4,6 SIGNATURE)	9m 3s
config (RELEASE 2,4,6 SIGNATURE LOW)	13m 28s
config (RELEASE 2,4,6 SIGNATURE MEDIUM)	17m 13s
config (RELEASE 8,10 SIGNATURE)	6m 25s
config (RELEASE 8,10 SIGNATURE LOW)	9m 16s
config (RELEASE 8,10 SIGNATURE MEDIUM)	12m 2s
config (MINSIZEREL 2,4,6 SIGNATURE)	15m 4s
config (MINSIZEREL 2,4,6 SIGNATURE LOW)	19m 41s
config (MINSIZEREL 2,4,6 SIGNATURE MEDIUM)	23m 59s
config (MINSIZEREL 8,10 SIGNATURE)	10m 35s
config (MINSIZEREL 8,10 SIGNATURE LOW)	13m 35s
config (MINSIZEREL 8,10 SIGNATURE MEDIUM)	16m 22s
	2h 46m 43s

Provide feedback