getting comment about constraint

How do you retrieve a comment on a constraint? For example, consider
the following table and comment:

create table people (
person_id serial primary key,
uid varchar(25) not null,
constraint uid_alphanumeric check (uid ~ '^[a-z0-9_]+$')
);

comment on constraint uid_alphanumeric on people
is 'UID may only contain letters, numerals, and underscores';

That code code creates a constraint within the "people" table named
"uid_alphanumeric". Furthermore, it creates a comment on that
constraint.

Given the name of the table and constraint, how would you retrieve the
comment? "Chapter 9. Functions and Operators" says that
obj_description(object_oid, catalog_name) returns a comment given the
object's OID and class name. How do I get the OID for the constraint?
It doesn't seem to be stored in pg_constraint.