This HOWTO shows how to create link to anchor across different articles.
At the moment only one way exists how to achieve it - by hand in HTML mode .
You need to switch to HTML mode and insert by hand HTML tag "A" with proper href attribute like this:
<a href="/docs/DOC-14883#Chapter_21">Article - Chapter 2.1</a>
The result is: Article - Chapter 2.1
If you'd like to use permanent link like in this example you can find it in Actions bar. Of course you can also use Human Friendly link from Table of Contents.