In Star Wars Episode V: The Empire Strikes Back, Yoda and Obi-Wan both encourage Luke to stay on Dagobah to finish his training. They beg him:
YODA: Luke, you must complete the training.
LUKE: I can’t keep the vision out of my head. They’re my friends. I’ve got to help them.
YODA: You must not go.
He refuses them and leaves for Bespin to save his friends.
The next time we see Yoda, in Star Wars Episode VI: Return of the Jedi, he tells Luke that he's dying. The conversation continues:
LUKE: But I need your help. I’ve come back to complete the training.
YODA: No more training do you require. Already know you that which you need.
LUKE: Then I am a Jedi.
YODA: Oh! Not yet. One thing remains... Vader. You must confront Vader. Then, only then, a Jedi will you be.
If the only thing left in Luke's training is to confront Vader, why are Yoda and Obi-Wan so concerned with him leaving to do exactly that?
If he already knows "that which he needs", why wouldn't they simply drop the bomb (Vader=Anakin) and just say "go get him, good luck"?
Was there some additional training that we didn't see? Possibly after Luke returned to Dagobah but before Yoda's death?