Trivial doc-patch for constraint description

Hi,

there's a paragraph in the ALTER TABLE reference page that reads:

DROP CONSTRAINT

This form drops constraints on a table. Currently, constraints on tables
are not required to have unique names, so there may be more than one
constraint matching the specified name. All matching constraints will be
dropped.

To my knowledge, it is no longer possible to create constraints with the
same name for the same relation. When you create a constraint and specify
the same name explictly, an error is raised. Implicit constraint creation
won't choose an existing name either and up to now you could not rename a
constraint. Renaming works with the patch I sent in a few hours ago but this
patch as well won't allow constraints with identical names on the same
relation.

The attached patch thus removes the note in the docs.

Joachim