Adding column_constraint description in ALTER TABLE synopsis

Hello,

As Amit Langot pointed out, the column_constraint definition is missing
whereas it is used in ALTER TABLE synopsis. It can be easily found in the
CREATE TABLE synopsis, but it's not very user friendly.

I simply copied/paste the column_constraint definition from the CREATE
TABLE synopsis to the ALTER TABLE synopsis. I also had to change the first
word "where" to "and" as it's not the first definition in that synopsis.

I choose to add it above table_constraint as column_constraint is used
before table_constraint.

The patch should apply to MASTER (or I messed up with git). I built and
tested it successfully on my laptop.

You will find enclosed my patch.

Regards,

Lætitia