Comma separated test case names are poorly documented

**Description**
In the [command line description](https://github.com/catchorg/Catch2/blob/master/docs/command-line.md#specifying-which-tests-to-run) for specifying which tests to run, it is not immediately clear how to OR multiple test case names.

I first assumed that I had to separate them by space (and I'm not the only one #1140). Then by reading that tags can have a comma as OR, I deduced that it might also work for test names.

I think it would be much clearer to explicitly say that test case names can be comma separated as well.