Subdocument Lookupin Exists documentation is incorrect

https://docs.couchbase.com/go-sdk/2.1/howtos/subdocument-operations.html#retrieving

Documentation for using ExistsSpec is incorrect. We have examples that show to use ContentAt to retrieve the result which will actually throw/return an error if the path doesn't exist. We need to be more clear about this and update the examples to use the Exists function for SDKs that follow this behaviour (there is a slight difference in behaviour across the SDKs).