When we first added XML doc comments to C#, we thought that using them would give us much more accurate documentation, as the developers would update the docs when they changed the code. This works pretty well for small libraries, but if you have a UE group that works on your docs, it's pretty inconvenient for them to have to check out your source files to work on docs.

So, we added an "include" syntax that lets you keep the docs in a separate file, which the compiler then reads to get the actual xml to use.

This is all documented here