This can be done in the following two ways -
cts:search( fn:collection("first-collection"), cts:word-query("marklogic"))
In this, the scope is changed from all the documents to documents in collection "first-collection" only.
In the second approach, use of cts:collection-query has been made. This should give better performance than the first approach.
cts:search( fn:doc(), cts:and-query(( cts:collection-query("first-collection"), cts:word-query("marklogic")))