[bsp] Add a comment about leaf marksurfaces

They should probably be cause leafsurfaces since they are the actual
surfaces of the leaf: ie, the faces of the leaf mesh if each leaf was
sub-sub-model.

include/QF/bspfile.h

@@ -201,6 +201,8 @@ typedef struct dleaf_s {
 	float       mins[3];			// for frustum culling
 	float       maxs[3];
 
+	// the list of marksurfaces is the surfaces that make up the effective
+	// mesh of the leaf
 	uint32_t    firstmarksurface;
 	uint32_t    nummarksurfaces;