doc: improve briefs #185

Job	Run time
build_emsdk (debug)	53s
build_emsdk (debug, -msimd128)	44s
build_emsdk (debug, -msse -msse2 -msimd128)	46s
build_emsdk (debug, -msse -msse2 -msse3 -msse4 -msimd128)	50s
build_emsdk (debugoptimized)	43s
build_emsdk (debugoptimized, -msimd128)	47s
build_emsdk (debugoptimized, -msse -msse2 -msimd128)	49s
build_emsdk (debugoptimized, -msse -msse2 -msse3 -msse4 -msimd128)	50s
build_emsdk (release)	45s
build_emsdk (release, -msimd128)	45s
build_emsdk (release, -msse -msse2 -msimd128)	47s
build_emsdk (release, -msse -msse2 -msse3 -msse4 -msimd128)	47s
build_emsdk (minsize)	49s
build_emsdk (minsize, -msimd128)	45s
build_emsdk (minsize, -msse -msse2 -msimd128)	44s
build_emsdk (minsize, -msse -msse2 -msse3 -msse4 -msimd128)	43s
	12m 27s

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