[igraph] Shortest Path documentation

[Matthew Walker]

Hello again!

Do I understand correctly that, for igraph_shortest_path() [1], the 
matrix "res" needs to be initialised before the call to the function?

If that's correct, then I think it would be great to add into the 
documentation a little note that says the matrix should indeed be 
initialised.  (Because it took me a loooong time to figure out why my 
test program was crashing, and initialisation of the matrix seemed to 
help! :o)

Cheers,

Matthew

[1] http://igraph.sourceforge.net/doc/html/igraph_shortest_paths.html