Documentation: amd-pstate: disambiguate user space sections

kernel test robot reported htmldocs warning:

Documentation/admin-guide/pm/amd-pstate.rst:343: WARNING: duplicate label admin-guide/pm/amd-pstate:user space interface in ``sysfs``, other instance in Documentation/admin-guide/pm/amd-pstate.rst

The documentation contains two sections with the same "User Space Interface
in ``sysfs``" title. The first one deals with per-policy sysfs and the
second one is about general attributes (currently only global attributes
are documented).

Disambiguate title text of both sections to fix the warning.

Link: https://lore.kernel.org/linux-doc/202302151041.0SWs1RHK-lkp@intel.com/
Fixes: b9e6a2d4

 ("Documentation: amd-pstate: introduce new global sysfs attributes")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Wyes Karny <wyes.karny@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>