Document tag name:value pair limitation.

Documents the limitation described in #1614. Please leave that issue open as a feature request.
Saying they are merged suggests that this is an active, intentional thing we've written code to do.  It's not...it's just that filters are represented internally as a hash and hashes don't allow multiple entries for the same key.  I think I'd write this as:

```
Filters are represented internally as a hash, which means that you can't
specify multiple filters for the same key. If you try to exclude `:name => 'foo'`
and `:name => 'bar'`, you will only end up excluding `:name => 'bar'`.
```