kernel/thread: Expand documentation of nominal_priority and current_priority

Aims to disambiguate why each priority instance exists a little bit.
While we're at it, also add an explanatory comment to UpdatePriority().

src/core/hle/kernel/thread.h

@@ -398,8 +398,14 @@ private:
     VAddr entry_point = 0;
     VAddr stack_top = 0;
 
-    u32 nominal_priority = 0; ///< Nominal thread priority, as set by the emulated application
-    u32 current_priority = 0; ///< Current thread priority, can be temporarily changed
+    /// Nominal thread priority, as set by the emulated application.
+    /// The nominal priority is the thread priority without priority
+    /// inheritance taken into account.
+    u32 nominal_priority = 0;
+
+    /// Current thread priority. This may change over the course of the
+    /// thread's lifetime in order to facilitate priority inheritance.
+    u32 current_priority = 0;
 
     u64 total_cpu_time_ticks = 0; ///< Total CPU running ticks.
     u64 last_running_ticks = 0;   ///< CPU tick when thread was last running