doc: Fix incorrect synopsis for ALTER PUBLICATION ... DROP ...

The synopsis for the ALTER PUBLICATION ... DROP ... command incorrectly
implied that a column list and WHERE clause could be specified as part of
the publication object. However, these options are not allowed for
DROP operations, making the documentation misleading.

This commit corrects the synopsis  to clearly show only the valid forms
of publication objects.

Backpatched to v15, where the incorrect synopsis was introduced.

Author: Peter Smith <smithpb2250@gmail.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/CAHut+PsPu+47Q7b0o6h1r-qSt90U3zgbAHMHUag5o5E1Lo+=uw@mail.gmail.com
Backpatch-through: 15

doc/src/sgml/ref/alter_publication.sgml

@@ -23,7 +23,7 @@ PostgreSQL documentation
 <synopsis>
 ALTER PUBLICATION <replaceable class="parameter">name</replaceable> ADD <replaceable class="parameter">publication_object</replaceable> [, ...]
 ALTER PUBLICATION <replaceable class="parameter">name</replaceable> SET <replaceable class="parameter">publication_object</replaceable> [, ...]
-ALTER PUBLICATION <replaceable class="parameter">name</replaceable> DROP <replaceable class="parameter">publication_object</replaceable> [, ...]
+ALTER PUBLICATION <replaceable class="parameter">name</replaceable> DROP <replaceable class="parameter">publication_drop_object</replaceable> [, ...]
 ALTER PUBLICATION <replaceable class="parameter">name</replaceable> SET ( <replaceable class="parameter">publication_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] )
 ALTER PUBLICATION <replaceable class="parameter">name</replaceable> OWNER TO { <replaceable>new_owner</replaceable> | CURRENT_ROLE | CURRENT_USER | SESSION_USER }
 ALTER PUBLICATION <replaceable class="parameter">name</replaceable> RENAME TO <replaceable>new_name</replaceable>
@@ -32,6 +32,11 @@ ALTER PUBLICATION <replaceable class="parameter">name</replaceable> RENAME TO <r
 
     TABLE [ ONLY ] <replaceable class="parameter">table_name</replaceable> [ * ] [ ( <replaceable class="parameter">column_name</replaceable> [, ... ] ) ] [ WHERE ( <replaceable class="parameter">expression</replaceable> ) ] [, ... ]
     TABLES IN SCHEMA { <replaceable class="parameter">schema_name</replaceable> | CURRENT_SCHEMA } [, ... ]
+
+<phrase>where <replaceable class="parameter">publication_drop_object</replaceable> is one of:</phrase>
+
+    TABLE [ ONLY ] <replaceable class="parameter">table_name</replaceable> [ * ] [, ... ]
+    TABLES IN SCHEMA { <replaceable class="parameter">schema_name</replaceable> | CURRENT_SCHEMA } [, ... ]
 </synopsis>
  </refsynopsisdiv>
 
@@ -55,8 +60,7 @@ ALTER PUBLICATION <replaceable class="parameter">name</replaceable> RENAME TO <r
    subscribing side in order to become effective. Note also that
    <literal>DROP TABLES IN SCHEMA</literal> will not drop any schema tables
    that were specified using <literal>FOR TABLE</literal>/
-   <literal>ADD TABLE</literal>, and the combination of <literal>DROP</literal>
-   with a <literal>WHERE</literal> clause is not allowed.
+   <literal>ADD TABLE</literal>.
   </para>
 
   <para>