Clarify sphere implicit equation

Resolves #1252

Author: hollasch

books/RayTracingInOneWeekend.html

@@ -774,9 +774,10 @@
 
     $$ x^2 + y^2 + z^2 = r^2 $$
 
-You can also think of this as saying that if a given point $(x,y,z)$ is on the sphere, then $x^2 +
-y^2 + z^2 = r^2$. If a given point $(x,y,z)$ is _inside_ the sphere, then $x^2 + y^2 + z^2 < r^2$,
-and if a given point $(x,y,z)$ is _outside_ the sphere, then $x^2 + y^2 + z^2 > r^2$.
+You can also think of this as saying that if a given point $(x,y,z)$ is on the surface of the
+sphere, then $x^2 + y^2 + z^2 = r^2$. If a given point $(x,y,z)$ is _inside_ the sphere, then
+$x^2 + y^2 + z^2 < r^2$, and if a given point $(x,y,z)$ is _outside_ the sphere, then
+$x^2 + y^2 + z^2 > r^2$.
 
 If we want to allow the sphere center to be at an arbitrary point $(C_x, C_y, C_z)$, then the
 equation becomes a lot less nice: