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.