perf docs: Improve help information in perf.txt

OPTIONS

-------

-h::

--help::

        Run perf help command.

-v::

--version::

        Display perf version.

-vv::

        Print the compiled-in status of libraries.

--exec-path::

        Display or set exec path.

--html-path::

        Display html documentation path.

-p::

--paginate::

        Set up pager.

--no-pager::

        Do not set pager.

--buildid-dir::

        Setup buildid cache directory. It has higher priority

        than buildid.dir config file option.

--list-cmds::

        List the most commonly used perf commands.

--list-opts::

        List available perf options.

--debugfs-dir::

        Set debugfs directory or set environment variable PERF_DEBUGFS_DIR.

--debug::

        Setup debug variable (see list below) in value

        range (0, 10). Use like:

          perf-event-open  - Print perf_event_open() arguments and

                             return value

--buildid-dir::

	Setup buildid cache directory. It has higher priority than

	buildid.dir config file option.

-v::

--version::

  Display perf version.

-h::

--help::

  Run perf help command.

DESCRIPTION

-----------

Performance counters for Linux are a new kernel-based subsystem