It has always (since the first commit) worked with relative paths, so
use the same wording as other parts of the documentation.
Author: Bruce Momjian
Discussion: https://postgr.es/m/CABUevExx-hm=cit+A9LeKBH39srvk8Y2tEZeEAj5mP8YfzNKUg@mail.gmail.com
<listitem>
<para>
- Specifies the file to be read. Must be an absolute path name.
+ Specifies the file to be read. Relative paths are relative to the
+ data directory.
Either <literal>filename</literal> or <literal>program</literal> must be
specified, but not both.
</para>