The documentation of pharo currently is much better than documentation for open source projects of similar size. Do not expect full or large documentation with such small community or class comments.
"See and discover yourself" is the approach to coding at large because in most cases whether you have a popular or non popular language documentation will always be a problem because developers generally don't like to write documentation.
However the best documentation by large for pharo is this mailing list and the slack channel. For example when I was learning NBOpenGL i literally bombarded this mailing list with questions and all of them got answered in just a few hours. For 4 years now, non of my questions have been left unanswered and I am one of those people that have no hesitation at all asking stupid questions :D
The irony here is that even though developers don't like writing documentation they absolutely LOOOOOVEEEEE answering questions :D