kernel/thread: Update thread processor ID flags

Adds the missing flags to the enum and documents them.

src/core/hle/kernel/svc.cpp

@@ -1776,7 +1776,7 @@ static ResultCode SetThreadCoreMask(Core::System& system, Handle thread_handle,
 
     if (core == OnlyChangeMask) {
         core = thread->GetIdealCore();
-    } else if (core >= Core::NUM_CPU_CORES && core != static_cast<u32>(-1)) {
+    } else if (core >= Core::NUM_CPU_CORES && core != static_cast<u32>(THREADPROCESSORID_DONT_UPDATE)) {
         LOG_ERROR(Kernel_SVC, "Invalid core specified, got {}", core);
         return ERR_INVALID_PROCESSOR_ID;
     }

src/core/hle/kernel/thread.h

@@ -30,7 +30,16 @@ enum ThreadPriority : u32 {
 };
 
 enum ThreadProcessorId : s32 {
-    THREADPROCESSORID_IDEAL = -2, ///< Run thread on the ideal core specified by the process.
+    /// Indicates that no particular processor core is preferred.
+    THREADPROCESSORID_DONT_CARE = -1,
+
+    /// Run thread on the ideal core specified by the process.
+    THREADPROCESSORID_IDEAL = -2,
+
+    /// Indicates that the preferred processor ID shouldn't be updated in
+    /// a core mask setting operation.
+    THREADPROCESSORID_DONT_UPDATE = -3,
+
     THREADPROCESSORID_0 = 0,   ///< Run thread on core 0
     THREADPROCESSORID_1 = 1,   ///< Run thread on core 1
     THREADPROCESSORID_2 = 2,   ///< Run thread on core 2