ftrace: Document that multiple function_graph tracing may have different times

The function graph tracer now calculates the calltime internally and for
each instance. If there are two instances that are running function graph
tracer and are tracing the same functions, the timings of the length of
those functions may be slightly different:

# trace-cmd record -B foo -p function_graph -B bar -p function_graph sleep 5
# trace-cmd report
[..]
bar: sleep-981 [000] ...1. 1101.109027: funcgraph_entry: 0.764 us | mutex_unlock(); (ret=0xffff8abcc256c300)
foo: sleep-981 [000] ...1. 1101.109028: funcgraph_entry: 0.748 us | mutex_unlock(); (ret=0xffff8abcc256c300)
bar: sleep-981 [000] ..... 1101.109029: funcgraph_exit: 2.456 us | } (ret=0xffff8abcc256c300)
foo: sleep-981 [000] ..... 1101.109029: funcgraph_exit: 2.403 us | } (ret=0xffff8abcc256c300)
bar: sleep-981 [000] d..1. 1101.109031: funcgraph_entry: 0.844 us | fpregs_assert_state_consistent(); (ret=0x0)
foo: sleep-981 [000] d..1. 1101.109032: funcgraph_entry: 0.803 us | fpregs_assert_state_consistent(); (ret=0x0)

Link: https://lore.kernel.org/all/[e-mail]/

Cc: Mark Rutland <[e-mail]>
Cc: Mathieu Desnoyers <[e-mail]>
Suggested-by: Masami Hiramatsu <[e-mail]>
Link: https://lore.kernel.org/[e-mail]
Signed-off-by: Steven Rostedt (Google) <[e-mail]>

Documentation/trace/ftrace.rst

···
 	to draw a graph of function calls similar to C code
 	source.
 
+	Note that the function graph calculates the timings of when the
+	function starts and returns internally and for each instance. If
+	there are two instances that run function graph tracer and traces
+	the same functions, the length of the timings may be slightly off as
+	each read the timestamp separately and not at the same time.
+
   "blk"
 
 	The block tracer. The tracer used by the blktrace user