[Mpi3-ft] Text tweaks

[Darius Buntinas]

New wording based on comments during the call.

-d

MPI_GROUP_VALIDATE is a process local operation that takes a snapshot
of the local list of known failed processes for the specified process
group. If any new failures were added to the local list (either since
the group was created or the last call to MPI_GROUP_VALIDATE), then
newfailures will be set to true, otherwise it will be set to false.

The following accessor functions are used to query the snapshot of the
local known failed processes taken with MPI_GROUP_VALIDATE.  Note the
snapshot only change when MPI_GROUP_VALIDATE is called.  This
means that queries of the state of a process using the following
accessor functions will always return the same value until
MPI_GROUP_VALIDATE is called again.

  Rationale. The constant state requirement between calls
  MPI_GROUP_VALIDATE and similar update validation operations allows
  the application to manage thread safety while querying the snapshot
  of local list of processes (maintained by the MPI
  implementation). (End of rationale.)