This method should be assumed to clear the data of the underlying array
(it will do this if the new limit > data.length).
This means it should only be called (directly) before writing into the
backing array.
It shouldn't be used as a shorthand for position=0, limit=x - those
should be two explicit method calls.
Most of these changes are no-ops, but they make the code more correct.
The TS SectionReader can't be easily changed to be 'safe', because it
relies on sectionData maintaining state between iterations of the while
loop. Instead I've added comments justifying the existing code.
PiperOrigin-RevId: 344515340
46598a46fd