Add some Nanos6 emu documentation

doc/emu_nanos6.txt

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+--- Nanos6 Emulation ---
+
+The Nanos6 emulator generates four different Paraver views, which are explained
+in this document.
+
+--- Task id ---
+
+The task id view represents the id of the Nanos6 task instance that is currently
+executing on each thread/cpu. This id is a monotonically increasing identifier
+assigned on task creation. Lower ids correspond to tasks created at an earlier
+point than higher ids.
+
+--- Task type ---
+
+Every task in Nanos6 contains a task type, which roughly corresponds to the
+actual location in the code a task was declared. For example if a function fn()
+is declared as a Nanos6 task, and it is called multiple times in a program,
+every created task will have a different id, but the same type.
+
+In the view, each type is shown with a label declared in the source with the
+label() attribute of the task. If no label was specified, one is automatically
+generated for each type.
+
+Note that in this view, event value is a hash function of the type label, so two
+distinct types (tasks declared in different parts of the code) with the same
+label will share event value and will hence be indistinguishable.
+
+--- MPI Rank ---
+
+Represents the current MPI rank for the currently running task in a thread or cpu.
+
+--- Subsystem ---
+
+Represents the internal Nanos6 subsystem each thread or cpu is currently
+running. Here is a summary of each possible value with its meaning:
+
+ - Null or black (value 0): Either the thread is idle (blocked because there is
+   no work) or the current subsystem is not instrumented
+ - "Scheduler: Waiting for tasks": Actively waiting for tasks inside the scheduler
+   subsystem, registered but not holding the scheduler lock
+ - "Scheduler: Serving tasks": Inside the scheduler lock, serving tasks to other
+   threads
+ - "Scheduler: Adding ready tasks": Adding tasks to the scheduler queues, but outside of
+   the scheduler lock
+ - "Task: Running": Executing user task code
+ - "Task: Spawning function": Registering a new spawn function (programmatically
+   created task)
+ - "Task: Creating": Creating a new task, through nanos6_create_task
+ - "Task: Submitting": Submitting a recently created task, through
+   nanos6_submit_task
+ - "Dependency: Registering": Registering a task's dependencies
+ - "Dependency: Unregistering": Releasing a task's dependencies because it has
+   ended
+ - "Blocking: Taskwait": Task is blocked while inside a taskwait
+ - "Blocking: Blocking current task": Task is blocked through the Nanos6
+   blocking API
+ - "Blocking: Unblocking remote task": Unblocking a different task using the
+   Nanos6 blocking API
+ - "Blocking: Wait For": Blocking a deadline task, which will be re-enqueued
+   when a certain amount of time has passed
+ - "Threading: Attached as external thread": External/Leader thread (which has
+   registered to Nanos6) is running
+