http://classic.musicbrainz.org/ws/1/release/8c555558-d59f-45ed-a714-b3206b56df3e?type=xml&inc=artist-rels
http://musicbrainz.org/ws/1/release/b39eb8c6-7e41-4fca-b521-1abff569844d?type=xml&inc=artist-rels+tracks+track-level-rels

Old

<relation type="Instrument" attributes="AcousticGuitar" target="c745b42b-1974-4e6e-93f7-4960008def75" begin="" end="">
<artist id="c745b42b-1974-4e6e-93f7-4960008def75" type="Person">
<name>Mark Kelly</name>
<sort-name>Kelly, Mark</sort-name>
<disambiguation>guitarist</disambiguation>
</artist>
</relation>

New

<relation direction="backward" target="c745b42b-1974-4e6e-93f7-4960008def75" type="Instrument" attributes="Acoustic Guitar">
<artist id="c745b42b-1974-4e6e-93f7-4960008def75">
<name>Mark Kelly</name>
<sort-name>Kelly, Mark</sort-name>
<disambiguation>guitarist</disambiguation>
</artist>
</relation>

The spaces must be removed, otherwise there is no way to tell what are multiple attributes and what is a single multi-word attribute.