Quite simply, some packages have much better or sometimes just bigger documentation than others.
If the doc is lacking or minimal, sometimes it’s useful to look at the source, and particularly the test files will often show how the authors expected it to be used.
The documentation for that particular package is on the page you linked to, but it’s just one flat file which you can see by scrolling down (README.md). Searching for squared will give relevant hits.
Perhaps you might want something like this:
julia> using Distances
julia> evaluate(SqEuclidean(),[1 2 3], [3 4 5])
12