trace-cmd: documentation fixes.

- Remove reference to default events from the overall workload
  documentation. It was, as of recently, outdated, and was also
  redundant, as the actual defaults will be in the parameter-specific
  documentation.
- Remove reference to Android-specific trace-cmd binary -- this was not
  true for a long time.
- Clarify that the on-host trace-cmd binary is now optional due to the
  report_on_target config.

wa/instrumentation/trace-cmd.py

@@ -40,13 +40,13 @@ class TraceCmdInstrument(Instrument):
 
     name = 'trace-cmd'
     description = """
-    trace-cmd is an instrument which interacts with Ftrace Linux kernel internal
+    trace-cmd is an instrument which interacts with ftrace Linux kernel internal
     tracer
 
     From trace-cmd man page:
 
-    trace-cmd command interacts with the Ftrace tracer that is built inside the
-    Linux kernel. It interfaces with the Ftrace specific files found in the
+    trace-cmd command interacts with the ftrace tracer that is built inside the
+    Linux kernel. It interfaces with the ftrace specific files found in the
     debugfs file system under the tracing directory.
 
     trace-cmd reads a list of events it will trace, which can be specified in
@@ -54,13 +54,8 @@ class TraceCmdInstrument(Instrument):
 
         trace_events = ['irq*', 'power*']
 
-    If no event is specified in the config file, trace-cmd traces the following
-    events:
-
-        - sched*
-        - irq*
-        - power*
-        - cpufreq_interactive*
+    If no event is specified, a default set of events that are generally considered useful
+    for debugging/profiling purposes will be enabled.
 
     The list of available events can be obtained by rooting and running the
     following command line on the device ::
@@ -93,13 +88,17 @@ class TraceCmdInstrument(Instrument):
               is happening in each case from trace-cmd documentation:
               https://lwn.net/Articles/341902/.
 
-    This instrument comes with an Android trace-cmd binary that will be copied
-    and used on the device, however post-processing will be done on-host and
-    you must have trace-cmd installed and in your path. On Ubuntu systems, this
-    may be done with::
+    This instrument comes with an trace-cmd binary that will be copied and used
+    on the device, however post-processing will be, by default, done on-host and you must
+    have trace-cmd installed and in your path. On Ubuntu systems, this may be
+    done with::
 
         sudo apt-get install trace-cmd
 
+    Alternatively, you may set ``report_on_target`` parameter to ``True`` to enable on-target
+    processing (this is useful when running on non-Linux hosts, but is likely to take longer
+    and may fail on particularly resource-constrained targets).
+
     """
 
     parameters = [
@@ -114,7 +113,7 @@ class TraceCmdInstrument(Instrument):
         Parameter('functions', kind=list_of_strings,
                   global_alias='trace_functions',
                   description="""
-                  Specifies the list of functions to be traced. 
+                  Specifies the list of functions to be traced.
                   """),
         Parameter('buffer_size', kind=int, default=None,
                   global_alias='trace_buffer_size',