A nice quality of in-code comments that a video session does not have is locality: the explanations are closed to the explainees, so hopefully they can evolve in synch. Another nice quality (shared with blog posts) is that it can be proposed by third-parties and crowd-sourced with moderate efficiency: you can help document the OCaml implementation by sending a pull-request with comments on the parts you fought to understand.
This has notably been done by Alain Frisch for the parsetree representation during the 4.01 development phase, and recently (4.03 development cycle, GPR#310) in types.mli and typedtree.mli by Frédéric Bour, Gabriel Radanne and Thomas Refis, with helpful feedback from Alain Frisch and Jacques Garrigue.
Anyone can help by submitting their own contribution to documentation-as-comments.