Let's look at the context:
Joh 16:12 “I still have many things to say to you, but you cannot bear them now.
Joh 16:13 When the Spirit of truth comes, he will guide you into all the truth, for he will not speak on his own authority, but whatever he hears he will speak, and he will declare to you the things that are to come.
Joh 16:14 He will glorify me, for he will take what is mine and declare it to you.
Joh 16:15 All that the Father has is mine; therefore I said that he will take what is mine and declare it to you. (ESV)
Jesus is speaking specifically to his disciples. He first says that there are many things he has to tell them but that they "cannot bear them now." This is what leads Jesus to speak of the Spirit guiding them "into all the truth." And what will that look like? The Spirit of truth will "not speak on his own authority, but whatever he hears he will speak." What will he tell the disciples? He will tell them of "the things that are to come" and "will take what is mine and declare it to you."
This strongly suggests that the Spirit of truth will guide the disciples into all the truth of those things which the disciples could not then bear. This is most likely what is given in the rest of the NT, in the disciples' writings and teachings, which were inspired by God.
Those verses were spoken to the disciples for a specific reason, namely, to prepare them to accept those other things they were going to be taught and given through the Holy Spirit, things which they were not yet ready for, things which Jesus had not yet told them.