You probably want to go and read the docs on the Realm project website
Most of the documentation for the project is contained inline as comments in our header files. The docs/source/ folder contains additional long-form articles such as our docs index page.
You can generate the docs locally by using the build.sh script at the root of this repository. This requires installation of appledoc.
sh build.sh docs
This will generate docs under docs/output/ and install the documentation in your Xcode.
You will also find an Apple docset package at the root of that that folder. You can view the docset locally with tools like Dash or in Xcode directly with e.g. Docs for XCode.