Two small string improvements
This changes the non-descript 'Change arc' to something more meaningful, and tries to improve the tool tip for the rendering tile size, which didn't tell a user what this value is useful for.
@Tavmjong : Is the tool tip's contents correct? Or can you help improve it in some other way? I think it would be important to make it clear to the user why (if ever) they should change the value, and what to expect from a change. @ede123 has summarized it for me, and we've tried to come up with something, but it might be that we didn't get it right.