DOCS-979 updated after finding a comment from Randall

Randall commented in email:

_active is probably more correctly thought of as _started -- things remain active (currently) even when they're yielded. At least in my limited test and understanding of the execution logic. _active only goes back to false during a reset/when the op is done.

I checked in the source and _active will only be set to false
if the op has not started or the op has completed, otherwise it
is always true even if the op has yielded to another op.
merge: DOCS-979