doc: Move ALTER TABLE IF EXISTS description to better place

It was previously mixed in with the description of ALTER TABLE
subcommands.  Move it to the Parameters section, which is where it is on
other reference pages.

pointed out by Amit Langote

doc/src/sgml/ref/alter_table.sgml

@@ -121,16 +121,6 @@ ALTER TABLE ALL IN TABLESPACE <replaceable class="PARAMETER">name</replaceable>
     </listitem>
    </varlistentry>
 
-   <varlistentry>
-    <term><literal>IF EXISTS</literal></term>
-    <listitem>
-     <para>
-      Do not throw an error if the table does not exist. A notice is issued
-      in this case.
-     </para>
-    </listitem>
-   </varlistentry>
-
    <varlistentry>
     <term><literal>SET DATA TYPE</literal></term>
     <listitem>
@@ -726,6 +716,16 @@ ALTER TABLE ALL IN TABLESPACE <replaceable class="PARAMETER">name</replaceable>
 
     <variablelist>
 
+     <varlistentry>
+      <term><literal>IF EXISTS</literal></term>
+      <listitem>
+       <para>
+        Do not throw an error if the table does not exist. A notice is issued
+        in this case.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry>
       <term><replaceable class="PARAMETER">name</replaceable></term>
       <listitem>