Obtaining gratis copy of the SQL specification

The official and final texts are only available by purchase.

The drafts, otoh, are often no more than a google search away. Google for SQL200n.zip and you should get pretty close.

UPDATE ...
SET (a, b, c) = (1, 2, 3) ;

Yes, your understanding is correct and this is perfectly legal SQL syntax. As @a_horse_with_no_name mentioned in the comments, you can also use it in conditions (WHERE, HAVING, CASE WHEN, ...).
Examples:

WHERE (a,b,c) = (1, 2, 3)

WHERE (a,b,c) >= (1, 2, 3)

WHERE (a,b,c) IN ((1, 2, 3), (1, 1, 1), (2, 2, 2), (4, 5, 6))

WHERE (a,b,c) BETWEEN (1, 2, 3) AND (4, 5, 6)

The inequality (including BETWEEN) conditions use the lexicographic ordering, so a value of (3, 0, 17) would result to true for the above 2nd and 4th examples.

It can also be used for swapping the values from 2 columns:

UPDATE ...
SET (a, b) = (b, a) ;

Not all databases have implemented this syntax however. As far as I know, only in PostgreSQL, all these work.

In MySQL, it works with =, >=, <=, <> and IN but neither with BETWEEN nor in an assignment (like UPDATE).

In Oracle none of the variations work but one can use this, if the subquery returns one row:

SET (a, b, c) = (SELECT expression_a, expression_b, expression_b FROM ...)

so for a simple update, it can be:

SET (a, b, c) = (SELECT 1, 2, 3 FROM dual)