In the Glossary page:
http://docs.mongodb.org/manual/reference/glossary/#term-data-center-awareness

*See also* members[n].tags and data center awareness.

The link to data center awareness is going to an anchor (#replica-set-data-center-awareness) that's not in the replication page.
http://docs.mongodb.org/manual/replication/#replica-set-data-center-awareness

It should be probably go to:
http://www.mongodb.org/display/DOCS/Data+Center+Awareness#DataCenterAwareness-Tagging%28version2.0%29