Improve documentation on Q3

[szarnyasg]

In the current presentation, the multi-source bidirectional BFS is explained with an example that finds pairs of nodes with a path having a length of 'exactly 3'. This might be misleading as it might not find these nodes if there are paths with a shorter length between them.

[marci543]

A detailed example in the third slide shows that

• all pairs reachable in 3 hops are found that weren't found before,
• some additional pairs within shorter distance are also found.

The latter doesn't cause a problem since they are not added again to the ones found in 2x1 hops.