[doc] build: Move NvFBC note into don't build warning #2345

Job	Run time
host-linux	50s
host-windows-cross	54s
client (gcc, g++, xdg-shell, Release)	1m 23s
docs	51s
client (gcc, g++, xdg-shell, Debug)	1m 20s
client (gcc, g++, libdecor, Release)	1m 37s
host-windows-native	1m 48s
module	45s
client (gcc, g++, libdecor, Debug)	1m 36s
obs (gcc)	59s
client (clang, clang++, xdg-shell, Release)	1m 41s
obs (clang)	45s
client (clang, clang++, xdg-shell, Debug)	2m 2s
client (clang, clang++, libdecor, Release)	2m 11s
client (clang, clang++, libdecor, Debug)	1m 23s
	20m 5s

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