Add documentation for a new way to reference Positional Parameters

A new way of referencing a positional parameter is by prefixing the number of the parameter with @. [ This is including the existing ways of referencing positional parameters ]

Change - https://docs.couchbase.com/server/current/guides/prep-statements.html#placeholders

Change "To add a positional parameter to a query, either enter a dollar sign $ followed by the number of the parameter, or enter a question mark ?"

to "To add a positional parameter to a query, either enter a dollar sign $ or @ sign followed by the number of the parameter, or enter a question mark ?"

Example :

SELECT * FROM airline WHERE id > @1;