Doc: improve description of dump/restore's --clean and --if-exists.

Try to make these option descriptions a little clearer for novices.
Per gripe from Attila Gulyás.

Discussion: https://postgr.es/m/169590536647.3727336.11070254203649648453@wrigleys.postgresql.org

doc/src/sgml/ref/pg_dump.sgml

@@ -170,11 +170,12 @@ PostgreSQL documentation
       <term><option>--clean</option></term>
       <listitem>
        <para>
-        Output commands to clean (drop)
+        Output commands to <command>DROP</command> all the dumped
         database objects prior to outputting the commands for creating them.
-        (Unless <option>--if-exists</option> is also specified,
+        This option is useful when the restore is to overwrite an existing
-        restore might generate some harmless error messages, if any objects
+        database.  If any of the objects do not exist in the destination
-        were not present in the destination database.)
+        database, ignorable error messages will be reported during
+        restore, unless <option>--if-exists</option> is also specified.
        </para>
 
        <para>
@@ -839,9 +840,11 @@ PostgreSQL documentation
       <term><option>--if-exists</option></term>
       <listitem>
        <para>
-        Use conditional commands (i.e., add an <literal>IF EXISTS</literal>
+        Use <literal>DROP ... IF EXISTS</literal> commands to drop objects
-        clause) when cleaning database objects.  This option is not valid
+        in <option>--clean</option> mode.  This suppresses <quote>does not
-        unless <option>--clean</option> is also specified.
+        exist</quote> errors that might otherwise be reported.  This
+        option is not valid unless <option>--clean</option> is also
+        specified.
        </para>
       </listitem>
      </varlistentry>

doc/src/sgml/ref/pg_dumpall.sgml

@@ -91,9 +91,12 @@ PostgreSQL documentation
       <term><option>--clean</option></term>
       <listitem>
        <para>
-        Include SQL commands to clean (drop) databases before
+        Emit SQL commands to <command>DROP</command> all the dumped
-        recreating them.  <command>DROP</command> commands for roles and
+        databases, roles, and tablespaces before recreating them.
-        tablespaces are added as well.
+        This option is useful when the restore is to overwrite an existing
+        cluster.  If any of the objects do not exist in the destination
+        cluster, ignorable error messages will be reported during
+        restore, unless <option>--if-exists</option> is also specified.
        </para>
       </listitem>
      </varlistentry>
@@ -324,9 +327,11 @@ PostgreSQL documentation
       <term><option>--if-exists</option></term>
       <listitem>
        <para>
-        Use conditional commands (i.e., add an <literal>IF EXISTS</literal>
+        Use <literal>DROP ... IF EXISTS</literal> commands to drop objects
-        clause) to drop databases and other objects.  This option is not valid
+        in <option>--clean</option> mode.  This suppresses <quote>does not
-        unless <option>--clean</option> is also specified.
+        exist</quote> errors that might otherwise be reported.  This
+        option is not valid unless <option>--clean</option> is also
+        specified.
        </para>
       </listitem>
      </varlistentry>

doc/src/sgml/ref/pg_restore.sgml

@@ -111,10 +111,12 @@ PostgreSQL documentation
       <term><option>--clean</option></term>
       <listitem>
        <para>
-        Clean (drop) database objects before recreating them.
+        Before restoring database objects, issue commands
-        (Unless <option>--if-exists</option> is used,
+        to <command>DROP</command> all the objects that will be restored.
-        this might generate some harmless error messages, if any objects
+        This option is useful for overwriting an existing database.
-        were not present in the destination database.)
+        If any of the objects do not exist in the destination database,
+        ignorable error messages will be reported,
+        unless <option>--if-exists</option> is also specified.
        </para>
       </listitem>
      </varlistentry>
@@ -580,9 +582,11 @@ PostgreSQL documentation
       <term><option>--if-exists</option></term>
       <listitem>
        <para>
-        Use conditional commands (i.e., add an <literal>IF EXISTS</literal>
+        Use <literal>DROP ... IF EXISTS</literal> commands to drop objects
-        clause) to drop database objects.  This option is not valid
+        in <option>--clean</option> mode.  This suppresses <quote>does not
-        unless <option>--clean</option> is also specified.
+        exist</quote> errors that might otherwise be reported.  This
+        option is not valid unless <option>--clean</option> is also
+        specified.
        </para>
       </listitem>
      </varlistentry>