doc: Add WITHOUT OVERLAPS to ALTER TABLE reference docs

Commit fc0438b4e8 documented WITHOUT OVERLAPS in the CREATE TABLE
docs, but not in ALTER TABLE.  This commit adds the new syntax to the
ALTER TABLE docs.

Author: Paul A. Jungwirth <pj@illuminatedcomputing.com>
Discussion: https://www.postgresql.org/message-id/57ea0668-5205-426e-b934-efc89f2186c2@illuminatedcomputing.com

doc/src/sgml/ref/alter_table.sgml

@@ -115,8 +115,8 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
 [ CONSTRAINT <replaceable class="parameter">constraint_name</replaceable> ]
 { CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ] |
   NOT NULL <replaceable class="parameter">column_name</replaceable> [ NO INHERIT ] |
-  UNIQUE [ NULLS [ NOT ] DISTINCT ] ( <replaceable class="parameter">column_name</replaceable> [, ... ] ) <replaceable class="parameter">index_parameters</replaceable> |
-  PRIMARY KEY ( <replaceable class="parameter">column_name</replaceable> [, ... ] ) <replaceable class="parameter">index_parameters</replaceable> |
+  UNIQUE [ NULLS [ NOT ] DISTINCT ] ( <replaceable class="parameter">column_name</replaceable> [, ... ] [, <replaceable class="parameter">column_name</replaceable> WITHOUT OVERLAPS ] ) <replaceable class="parameter">index_parameters</replaceable> |
+  PRIMARY KEY ( <replaceable class="parameter">column_name</replaceable> [, ... ] [, <replaceable class="parameter">column_name</replaceable> WITHOUT OVERLAPS ] ) <replaceable class="parameter">index_parameters</replaceable> |
   EXCLUDE [ USING <replaceable class="parameter">index_method</replaceable> ] ( <replaceable class="parameter">exclude_element</replaceable> WITH <replaceable class="parameter">operator</replaceable> [, ... ] ) <replaceable class="parameter">index_parameters</replaceable> [ WHERE ( <replaceable class="parameter">predicate</replaceable> ) ] |
   FOREIGN KEY ( <replaceable class="parameter">column_name</replaceable> [, ... ] ) REFERENCES <replaceable class="parameter">reftable</replaceable> [ ( <replaceable class="parameter">refcolumn</replaceable> [, ... ] ) ]
     [ MATCH FULL | MATCH PARTIAL | MATCH SIMPLE ] [ ON DELETE <replaceable class="parameter">referential_action</replaceable> ] [ ON UPDATE <replaceable class="parameter">referential_action</replaceable> ] }