Restriction on comments

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/15/sql-comment.html
Description:

I've recently been converting some DML which was intended for BigQuery to
use PostgreSQL 14.  Using a python client (using sqlalchemy and the
postgresql+psycopg2 driver), I had successfully created a table and was
trying to add comments to the columns.  All of this was previously done in a
single statement on BigQuery, but requires separate CREATE TABLE and COMMENT
ON COLUMN statements for PostgreSQL.

For one of the comment statements I received this error: "dict is not a
sequence".  It turns out the culprit was that one of the comments had a '%'
character.  I'm not familiar with restrictions on strings in PostgreSQL, but
it would be helpful to have a list of these.