Update descriptions in pose_graph.h (#977)
Update descriptions in pose_graph.h (#977)

Remove "frozen", since deserialized trajectories can be unfrozen since #891.
wohe/cartographer,wohe/cartographer,cartographer-project/cartographer,googlecartographer/cartographer,cartographer-project/cartographer,larics/cartographer,cartographer-project/cartographer,damonkohler/cartographer,cartographer-project/cartographer,larics/cartographer,wohe/cartographer,larics/cartographer,googlecartographer/cartographer,googlecartographer/cartographer,damonkohler/cartographer,damonkohler/cartographer,larics/cartographer,damonkohler/cartographer,wohe/cartographer,googlecartographer/cartographer