Update the list of avialble nodes

Hello, it would be nice to upgrade the list of nodes in the "Nodes Library", plenty are not described (sleep, alwaysfailed, if-then-else, while, switch ...) or described in other parts (parallel, actions ...), could be interesting to add a "leaf nodes" section.

Yeah we can find them back reading the code but is not this goal of a doc ? :) I'm too beginner with BT to create a PR right now.