Document tag name:value pair limitation.

Documents the limitation described in #1614. Please leave that issue open as a feature request.
Or without mentioning the implementation details:

```
Filters that use the same `name` in a `name:value` pair will be overwritten,
so if you try to exclude `:name => 'foo'` and `:name => 'bar'`, you will only
end up excluding `:name => 'bar'`.
```