While some devs are nice enough to document all possible values for each setting in the default config file itself, often I end up copying random dot files I find online to find the settings I want.
Since config files are non-standard this is probably wishful thinking, but I find myself wanting a Sublime Text or VS Code -like way to edit config files. Does one exist? Maybe as a vim plugin?
About the content, as you said, there is no standard. Each application uses its own configuration as it needs. If there is no online manual, there should at least be a CLI manual (man myapp) or help argument (myapp --help or myapp -h).
About the format, there are several (properties, JSON, XML, YAML…) and each developer is free to use the one they want. Though at the end of the day, they’re all text based, so a text editor with some content assist may help formatting those.
Then, it all depends on the application. Do note that some have separate GUIs which may have configuration menus.