Amend sections indicating XATTR manipulation is possible.

In the section on "OPTIONS" here:

https://docs.couchbase.com/server/current/n1ql/n1ql-language-reference/insert.html#insert-select

change "Only the expiration attribute has any effect; any other attributes are ignored." to "Only the listed attributes have any effect; any other attributes are ignored."

and after the item "expiration" add:

xattrs 

An object containing at the top-level extended attribute names and their valid JSON content.  Note: The object does not replace all defined extended attributes and only lists those to be added or updated, and only complete values can be provided; you cannot specify only nested fields to mutate as the complete value is assigned at the top level.

(An example for clarity:  supposing there is an existing XATTR called "a" with a value {"b":1}. An  UPSERT with options: {"xattr":{"a":{"c":1}} would replace the XATTR "a"'s value with {"c":1}.)

Update the syntax diagrams and explanation here:

https://docs.couchbase.com/server/current/n1ql/n1ql-language-reference/update.html#set-clause

indicating that in addition to:

   SET meta().expiration = expiration
,
   SET meta().xattrs.<xattr>[.path] = <value>

is valid syntax too.

Perhaps changing the definition of "meta" to something like:

A META().expiration expression specifying the expiration property of the document being updated or META().xattrs.<xattr> expression specifying the extended attribute (XATTR) of the document being updated.  Individual fields in composite XATTR values may be referenced directly via the nested path.

Similarly here:

https://docs.couchbase.com/server/current/n1ql/n1ql-language-reference/update.html#unset-clause

indicate that:

   UNSET meta().xattrs.<xattr>[.path]

is valid syntax.