[documentation] Missing usage info for getting tag var definitions

**Describe the bug**
A clear and concise description of what the bug is.

You're [missing documentation](https://mikefarah.gitbook.io/yq/usage/xml#parse-xml-attributes) for how to use XML.

```xml
<?xml version="1.0" encoding="UTF-8"?>
<cat legs="4">
  <legs>7</legs>
</cat>
```

**How do you get `legs=4`?**
