-- Additionally, the start and end handles must return an int and accept the same parameters as init.

Actually both of them could accept no parameters as shown in MethodHandles.countedLoop(MH, MH, MH) lambdaman example

-- Implementation requirements:
// assume MH_increment and MH_lessThan are handles to x+1 and x<y of type int

'x < y of type int' looks a little bit strange. Maybe it would be good to clarify this a little bit.

+ It is hard to deduce resulting handle parameter list in the case of absent 'init' function. The situation is the same as in JDK-8150964 comment