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/[email protected]/

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

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