Add some comments to GfxMode / CurrentGfxModeGuard.

src/gfxmode.h

@@ -19,6 +19,7 @@
 #ifndef GFXMODE_H_INCLUDED
 #define GFXMODE_H_INCLUDED
 
+// Holds settings about a graphics mode.
 class GfxMode
 {
 public:
@@ -46,19 +47,40 @@ private:
   int m_scaling;
 };
 
+// Tries a new graphics mode, and restores it back in the
+// destructor. If the keep() method is used, the mode is not
+// restored.
 class CurrentGfxModeGuard
 {
 public:
+  // Saves the current graphics mode as "the original" one.
   CurrentGfxModeGuard();
+
+  // Restores the graphics mode to its original in case we have used
+  // tryGfxMode() and we did not call keep(). After calling keep(),
+  // this destructor will do just nothing.
   ~CurrentGfxModeGuard();
 
+  // Changes the current graphics mode. Returns true if the mode was
+  // changed successfully. If the function cannot change the new mode
+  // and cannot restore the original one, it finishes the program
+  // (with exit()).
   bool tryGfxMode(const GfxMode& newMode);
+
+  // Keeps the last graphics mode set with tryGfxMode(). Indicates
+  // that the destructor should not restore the graphics mode to the
+  // original one.
   void keep();
 
+  // It is the original graphics mode configured when the constructor
+  // of this class was called.
   const GfxMode& getOriginal() const;
 
 private:
+  // The original gfx mode.
   GfxMode m_oldMode;
+
+  // True if we should keep any mode set with tryGfxMode() method.
   bool m_keep;
 };