Does "ON UPDATE" for foreign keys require index?

Hi. Say I have column A.b_id which references B.id (which is a primary
key) and as such it is declared as a foreign key constraint. A.b_id has
no index because it doesn't need one. What happens when table B's rows
are modified (but never deleted)? Will PG still have have to scan A fo
find A.b_id to do nothing with it? )) B.id itself is never modified,
it's just a normal serial value typically used for id's.

The docs are a bit ambiguous:

> Since a DELETE of a row from the referenced table *or an **UPDATE**of
> a referenced column* will require a scan of the referencing table for
> rows matching the old value, it is often a good idea to index the
> referencing columns too. Because this is not always needed, and there
> are many choices available on how to index, declaration of a foreign
> key constraint does not automatically create an index on the
> referencing columns.
>
>

https://www.postgresql.org/docs/9.6/ddl-constraints.html