Rearrange documentation paragraph describing pg_relation_size().

Break the list of available options into an <itemizedlist> instead of
inline sentences.  This is mostly motivated by wanting to ensure that the
cross-references to the FSM and VM docs don't cross page boundaries in PDF
format; but it seems to me to read more easily this way anyway.  I took the
liberty of editorializing a bit further while at it.

Per complaint from Magnus about 9.0.18 docs not building in A4 format.
Patch all active branches so we don't get blind-sided by this particular
issue again in future.

doc/src/sgml/func.sgml

@@ -15098,18 +15098,34 @@ postgres=# SELECT * FROM pg_xlogfile_name_offset(pg_stop_backup());
    </para>
    <para>
-    <function>pg_relation_size</> accepts the OID or name of a table, index or
-    toast table, and returns the on-disk size in bytes. Specifying
-    <literal>'main'</literal> or leaving out the second argument returns the
-    size of the main data fork of the relation. Specifying
-    <literal>'fsm'</literal> returns the size of the
-    Free Space Map (see <xref linkend="storage-fsm">) associated with the
-    relation. Specifying <literal>'vm'</literal> returns the size of the
-    Visibility Map (see <xref linkend="storage-vm">) associated with the
-    relation.  Note that this function shows the size of only one fork;
-    for most purposes it is more convenient to use the higher-level
-    functions <function>pg_total_relation_size</> or
-    <function>pg_table_size</>.
+    <function>pg_relation_size</> accepts the OID or name of a table, index
+    or toast table, and returns the on-disk size in bytes of one fork of
+    that relation.  (Note that for most purposes it is more convenient to
+    use the higher-level functions <function>pg_total_relation_size</>
+    or <function>pg_table_size</>, which sum the sizes of all forks.)
+    With one argument, it returns the size of the main data fork of the
+    relation.  The second argument can be provided to specify which fork
+    to examine:
+    <itemizedlist spacing="compact">
+     <listitem>
+      <para>
+       <literal>'main'</literal> returns the size of the main
+       data fork of the relation.
+      </para>
+     </listitem>
+     <listitem>
+      <para>
+       <literal>'fsm'</literal> returns the size of the Free Space Map
+       (see <xref linkend="storage-fsm">) associated with the relation.
+      </para>
+     </listitem>
+     <listitem>
+      <para>
+       <literal>'vm'</literal> returns the size of the Visibility Map
+       (see <xref linkend="storage-vm">) associated with the relation.
+      </para>
+     </listitem>
+    </itemizedlist>
    </para>
    <para>