Provide default values & comments for FRE config

Google-Bug-Id:561657274
PiperOrigin-RevId: 981384082
Change-Id: I7bf214d320336ccdfa60b622c1000eb31729a7ef

tlbmc/thermal_config.proto

diff --git a/tlbmc/thermal_config.proto b/tlbmc/thermal_config.proto
index 9492f00..8f8bd28 100644
--- a/tlbmc/thermal_config.proto
+++ b/tlbmc/thermal_config.proto
@@ -466,13 +466,20 @@
   // Target phase margin `\phi_m` (degrees).
   double target_phase_margin_deg = 2;
   // Perturbation signal amplitude `A_{total}`.
-  double sine_amplitude = 3;
+  double sine_amplitude = 3 [default = 5];
   // Duration of the tuning process in seconds.
-  uint32 tuning_duration_sec = 4;
-  double max_output = 5;
-  double min_output = 6;
-  double nominal_output = 7;
-  int32 loop_sign = 8;
+  uint32 tuning_duration_sec = 4 [default = 100];
+  // The maximum output value (e.g., fan PWM) in the domain for the PID
+  // controller.
+  double max_output = 5 [default = 100];
+  // The minimum output value (e.g., fan PWM) in the domain for the PID
+  // controller.
+  double min_output = 6 [default = 0];
+  // Baseline control output `u_0` that the perturbation is applied around.
+  double nominal_output = 7 [default = 50];
+  // Sign of the tuned coefficient gains. Only `1` (positive loop gain) and
+  // `-1` (negative loop gain) are meaningful values.
+  int32 loop_sign = 8 [default = 1];
 }
 
 message PidTunerConfig {