[doc] faq.rst: Reword libdecor package installation #2301

Job	Run time
docs	57s
host-linux	30s
host-windows-cross	1m 8s
client (gcc, g++, xdg-shell, Release)	1m 21s
client (gcc, g++, xdg-shell, Debug)	54s
client (gcc, g++, libdecor, Release)	2m 50s
client (gcc, g++, libdecor, Debug)	1m 29s
client (clang, clang++, xdg-shell, Release)	1m 43s
client (clang, clang++, xdg-shell, Debug)	1m 16s
client (clang, clang++, libdecor, Release)	1m 28s
client (clang, clang++, libdecor, Debug)	1m 27s
module	15s
host-windows-native	1m 51s
obs (gcc)	45s
obs (clang)	1m 13s
	19m 7s

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