more docs and unit tests #sonar #259

Job	Run time
avr (c99)	1m 38s
optimizations (clang, Release)	1m 6s
debug (clang)	3m 21s
arm (c99)	1m 32s
avr (c11)	1m 50s
optimizations (clang, MinSizeRel)	1m 6s
arm (c11)	2m 4s
debug (gcc)	3m 12s
avr (gnu99)	1m 48s
arm (gnu99)	1m 25s
avr (gnu11)	1m 32s
arm (gnu11)	1m 55s
optimizations (gcc, Release)	1m 20s
style_check	9s
optimizations (gcc, MinSizeRel)	1m 9s
sonar	1m 53s
	27m 0s

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