drm/xe: Add missing documentation of rpa_freq

While at it, already adjust the rpe_freq frequency, to highlight
that both are calculated by PCODE at runtime.

Fixes: c6aac2f ("drm/xe: Introduce the RPa information")
Cc: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Link: https://lore.kernel.org/r/20250521165146.39616-4-rodrigo.vivi@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

Author: rodrigovivi

drivers/gpu/drm/xe/xe_gt_freq.c

@@ -36,7 +36,10 @@
  * - act_freq: The actual resolved frequency decided by PCODE.
  * - cur_freq: The current one requested by GuC PC to the PCODE.
  * - rpn_freq: The Render Performance (RP) N level, which is the minimal one.
+ * - rpa_freq: The Render Performance (RP) A level, which is the achiveable one.
+ *   Calculated by PCODE at runtime based on multiple running conditions
  * - rpe_freq: The Render Performance (RP) E level, which is the efficient one.
+ *   Calculated by PCODE at runtime based on multiple running conditions
  * - rp0_freq: The Render Performance (RP) 0 level, which is the maximum one.
  *
  * device/tile#/gt#/freq0/<item>_freq *read-write* files: