Move doxygen comment to header file.

src/Song.cpp

@@ -1151,9 +1151,6 @@ bool Song::HasEdits( StepsType st ) const
 	return false;
 }
 
-/* Return false if the song should not be displayed for selection in normal
- * gameplay (but may still be available in random selection, during extra
- * stages, or in other special conditions). */
 bool Song::NormallyDisplayed() const
 {
 	return UNLOCKMAN == NULL || !UNLOCKMAN->SongIsLocked(this);

src/Song.h

@@ -389,6 +389,12 @@ public:
 
 	void SetEnabled( bool b ) { m_bEnabled = b; }
 	bool GetEnabled() const { return m_bEnabled; }
+	/**
+	 * @brief Determine if the song should be shown on the MusicWheel normally.
+	 *
+	 * Songs that are not displayed normally may still be available during
+	 * random selection, extra stages, or other special conditions.
+	 * @return true if displayed normally, false otherwise. */
 	bool NormallyDisplayed() const;
 	bool ShowInDemonstrationAndRanking() const;