Note that descriptions are in DocBook format, so it's better to write something like:
```
This is in the format of 
<link xlink:href="http://www.postgresql.org/docs/9.2/static/auth-pg-hba-conf.html">the 
<filename>pg_hba.conf</filename> configuration file</link>.
```
or something like that. That way it shows up as an actual link in the NixOS manual.