There is no
tracker/docs/reference/libtracker-module/xml/writing-an-indexer-plugin.sgml file on svn. A missing commit ?

Build log:

cd ./html && gtkdoc-mkhtml
libtracker-module ../libtracker-module-docs.sgml
warning: failed to load external entity
"../xml/writing-an-indexer-plugin.sgml"
../libtracker-module-docs.sgml:56: parser error : Failure to process
entity writing-an-indexer-plugin
      &writing-an-indexer-plugin;
                                 ^
../libtracker-module-docs.sgml:56: parser error : Entity
'writing-an-indexer-plugin' not defined
      &writing-an-indexer-plugin;
                                 ^