It is not documented (thus, probably not supported), but you can do this using the same base URL as the couchbase bucket API (see table 4). The complete path is:
...hostname:8091/pools/default/buckets/{bucketname}/docs/{docid}
To confirm this, just go to your couchbase web console, and use your builtin browser developer tools to check all network queries, and hit save button. You can verify this is the actual REST endpoint that the couchbase web console hits when you open or save a document.
It uses basic HTTP auth. But, instead of bucket password it needs Admin credentials.
Important: You should not rely on this for high performance operations. Performance on number of gets and sets operations using this undocumented REST endpoint is way too slow compared to couchbase official SDK which is optimized protocol that does not carry the HTTP overhead.