[Libmesh-users] small doc and efficiency update for ex 3 and 4

[edgar <edg...@cr...>]

Hi,

I am humbly sharing something which I think would improve the 
documentation and the logic of examples 3 and 4 a bit. I think that this 
would apply to other examples as well. (I was planning to keep learning 
from the examples, and have a more substantial contribution at the end, 
but it has been like a month since I last touched libMesh, and I it 
seems that I am going to be very busy in the next couple of months).

Thanks.