Documentation regarding TASK_LOST is misleading

Our protos describe TASK_LOST as a terminal state [1] [2].

A task might go from TASK_LOST to TASK_RUNNING or another state if Mesos is not using a strict register, so the documentation is misleading.

Marathon used to assume that TASK_LOST was a terminal past and that resulted in production pain for some users.

We should update the documentation to make the life of frameworks developers a bit better =).