The documentation is always incomplete. Documentation describes intended behaviour. Source code describes actual behaviour. When they diverge, only one is authoritative.
Reading source code does several things simultaneously: resolves confusing documentation, surfaces undocumented options, reveals failure modes the documentation does not mention, and improves your own code by exposing you to how expert engineers structure complex systems.
When something surprises me, I go to the source before going to Stack Overflow. The source is always right.
Read the source of libraries and frameworks you depend on in production. Not obsessively — strategically. When something surprises you, go to the source.
— Dick Bassey | DevDick | 2022