seems to have disappeared ...
Newer versions of Nexus include this documentation (at least 2.0.4-1). Replace nexus.xwiki.org with a different host name, for example. hostname of your nexus company server (you may have to abandon the / nexus part):
Http: // [of your own nexus] /nexus/nexus-core-documentation-plugin/core/docs/index.html
[ EDIT ]
found an API document in a public instance of Nexus 2.1 here
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