Improve documentation around multivariate flags

[matthewelwell]

Currently, when trying to understand multivariate flags / A/B/n tests, there is a little bit of an infinite loop in our documentation, with no location giving a clear indication of what a multivariate flag is, or what the constraints are.

The key issue is that the Experimentation (A/B testing) page links out to the Basic Flag Management page for 'a basic understanding of multivariate flags', however, there is no information on that page on the topic.

We have a few options to deal with this:

1. Add a short section to the Basic Flag Management page on multivariate flags
2. Add some basic information about multivariate flags to the Experimentation page.

I think that I am in favour of option 1 here.

1 key piece of information that we are missing is the fact that multivariate flags only function when identifying users. See the section here courtesy of the wayback machine, or an old tag in Github here where we used to have this information.