There are very many rsyslog versions out in the wild. Each of these versions have different capabilities and consequently there is also different documentation required to match an exact version.
The rsyslog site always contains documentation for the latest development version (sometimes even features that just reside in git). While chances are good that the online doc can be used to solve a specific question, this is not guaranteed. Most importantly, older versions may have quite different rsyslog.conf formats that they support. As such, folks tend to ask how they can obtain a version of the documentation that exactly matches their version.
The answer is quite straightforward: most of the doc on the site comes from rsyslog’s project doc set, and that doc set is contained in each release tarball. So if you have e.g. 5.8.10 installed and want it’s doc, simply go ahead and download the relevant tarball. The full doc is contained in the "./doc" subdirectory. It’s in HTML format, so all you need to do is to point your browser at ./doc/index.html.
In many distributions it is even simpler, as there is a package rsyslog-doc (or similarly named), which contains that exact doc folder.