However, Isabelle/HOL/Isar allows proofs to be written in a very human readable way, so the "documentation" I use is the existing library and how things are done there. There is a convenient Ctrl+left click action on terms to jump to their definition. That is very helpful to navigate unknown libraries. In comparison, I couldn't read and make sense the proofs I've seen in Coq.
There is a learning curve with Isabelle/HOL/Isar. It can be extremely frustrating to not know how to express "simple" arguments or definition. But for someone familiar with manual "formal" proofs in mathematics, most can be learned in some hours under guidance, like in a seminar on a weekend with a couple of hours on each day.
I'm learning rather autodidactly by trial and error. I think there would be value in a series of blog posts about "This is how you define a group" or "This is how you reason in nonclassical logic" or "This is how to proof the fundamental theorem of Algebra" all applied to Isabelle/HOL/Isar. There are often multiple ways to define things, and to be honest, I don't know the specific advantages and disadvantages. For example, I think you can introduce rings as type classes or as locales or using axiomatizations, and possibly more. This is one example of stuff I don't know how to manage in the system, but there is already a lot of structure defined, and I can reuse that.