modules/wayland: document that scan_xml can take any protocol. #16656

Job	Run time
Arch Linux	19m 59s
Arch / PyPy	20m 1s
CUDA (on Arch)	7m 23s
Ubuntu Rolling (gcc, g++)	19m 14s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	13m 24s
Fedora	19m 37s
Gentoo	21m 55s
OpenSUSE	16m 15s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	13m 41s
Ubuntu Bionic	15m 13s
Ubuntu Rolling (clang, clang++)	22m 5s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	15m 33s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	13m 17s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	13m 5s
	3h 50m 42s

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