Is there any documentation somewhere about microsecond as a variable of rotation (such as radiants or deegres),
There is no documentation as to the relationship between the number of microseconds and degrees of rotation because each type of servo will vary as to how far it can rotate, hence the suggestions to try the extremes of servo.write() and writeMicroseconds(). Specialist servos, such as sail winches, may turn through 360 degrees or more from one end of their travel to another whilst when using continuous rotation servos the parameters used in the functions control the speed and direction of rotation rather than the angle.
From what you say your servo rotates 180 degrees end to end between servo.write0); and servo.write(180); What does it do with writeMicroseconds(1000); and writeMicroseconds(2000); ?