To add full-text search capabilities to a document database, developers can use Couchbase's Search API. The process involves indexing and analyzing text in each document, then sending a query string to the server to find matches. Developers can create a simple text search function by providing a search term, specifying the search index to use, declaring the query type (e.g., queryString), assembling the parts together, and sending it to the server. The results are returned with a list of matching document IDs and ranking scores. To implement this in a Node.js application, developers can use the Couchbase SDK and Express framework to create routes for searching documents by term or phrase. The search query API supports various options, such as wildcards, prefixes, and fuzzy matching, allowing developers to fine-tune their searches. With Couchbase's full-text search capabilities, developers can build powerful search-based applications with ease.