Generally, any documentation you would see on GitHub is also in the library folder on your computer, since it is those files that provide the content displayed on GitHub.
There are two exceptions:
On many GitHub repositories, you will see at tab at the top of the GitHub pages labeled "Wiki". Often you will find this tab does nothing because there is no wiki but the repository owner is too lazy/incompetent to disable the useless "red herring" wiki tab. However, sometimes you will discover a hidden treasure trove of documentation in the wiki. You would expect a link in the readme to the wiki but all too often I have found that nobody was considerate enough to do so.
At the top of the home page of the repository there is the repository description. Sometimes this contains a link to documentation on an external website. Again, you'd expect that link to also be provided in the readme, but that doesn't always happen.
Some libraries have doxygen documentation that is generated automatically from the source code. This documentation will be in HTML files in a subfolder (I think it's usually called "doc") of the repository and often there is no mention of their existence in the readme. Ideally, there would be manually created documentation text in specially formatted comments in the source code but often the developers don't make that effort and just let doxygen generate the bare minimum skeleton of the documentation from the code itself, which I find to be completely useless. However, with a reasonable amount of effort a considerate developer can produce useful documentation via doxygen.
If you're really desperate, you could check the url value in the library.properties file (if present) in the root folder of the repository. Usually this URL will just point to the library repository, but it might also point to some external website with documentation. There also might be links in the heading of the source files, most likely the .h file.