SELECT ... FOR UPDATE OF

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/13/sql-select.html
Description:

The FOR UPDATE clause of the SELECT statement can be followed by an OF
clause, but this is never explained at all in the documentation, as far as I
can tell. What does it do? This needs to be written down.

For example, when attempting to use the OF clause in a particular query, I
get the error "FOR UPDATE must specify unqualified relation names". The
documentation doesn't provide enough information for me to understand what I
have done wrong, so I will have to use trial and error to solve my problem.