filterExternalDocs = function(docs) {
  return _.reject(docs, function (doc) {
    if (!!doc.external) {
      externalTypes[doc.name] = doc.see[0];
      return true;
    }
    return false;
  });
}
Filters out types with @external annotations and adds their @see link to the
externalTypes table

@param {Array.<Object>} docs The jsdoc list.