perf report: Add documentation for dynamic sort keys

	And default sort keys are changed to comm, dso_from, symbol_from, dso_to

	and symbol_to, see '--branch-stack'.

	If the data file has tracepoint event(s), following (dynamic) sort keys

	are also available:

	trace, trace_fields, [<event>.]<field>[/raw]

	- trace: pretty printed trace output in a single column

	- trace_fields: fields in tracepoints in separate columns

	- <field name>: optional event and field name for a specific field

	The last form consists of event and field names.  If event name is

	omitted, it searches all events for matching field name.  The matched

	field will be shown only for the event has the field.  The event name

	supports substring match so user doesn't need to specify full subsystem

	and event name everytime.  For example, 'sched:sched_switch' event can

	be shortened to 'switch' as long as it's not ambiguous.  Also event can

	be specified by its index (starting from 1) preceded by the '%'.

	So '%1' is the first event, '%2' is the second, and so on.

	The field name can have '/raw' suffix which disables pretty printing

	and shows raw field value like hex numbers.  The --raw-trace option

	has the same effect for all dynamic sort keys.

	The default sort keys are changed to 'trace' if all events in the data

	file are tracepoint.

-F::

--fields=::

	Specify output field - multiple keys can be specified in CSV format.