Fix documentation oversights about pageinspect and initialization fork.

The initialization fork was added in 9.1, but has not been taken into
consideration in documents of get_raw_page function in pageinspect and
storage layout. This commit fixes those oversights.

get_raw_page can read not only a table but also an index, etc. So it
should be documented that the function can read any relation. This commit
also fixes the document of pageinspect that way.

Back-patch to 9.1 where those oversights existed.

Vik Fearing, review by MauMau

doc/src/sgml/pageinspect.sgml

@@ -28,11 +28,12 @@
     <listitem>
      <para>
       <function>get_raw_page</function> reads the specified block of the named
-      table and returns a copy as a <type>bytea</> value.  This allows a
+      relation and returns a copy as a <type>bytea</> value.  This allows a
       single time-consistent copy of the block to be obtained.
       <replaceable>fork</replaceable> should be <literal>'main'</literal> for
-      the main data fork, or <literal>'fsm'</literal> for the free space map,
-      or <literal>'vm'</literal> for the visibility map.
+      the main data fork, <literal>'fsm'</literal> for the free space map,
+      <literal>'vm'</literal> for the visibility map, or <literal>'init'</literal>
+      for the initialization fork.
      </para>
     </listitem>
    </varlistentry>

doc/src/sgml/storage.sgml

@@ -265,8 +265,8 @@ The <function>pg_relation_filepath()</> function shows the entire path
 as a substitute for remembering many of the above rules.  But keep in
 mind that this function just gives the name of the first segment of the
 main fork of the relation &mdash; you may need to append a segment number
-and/or <literal>_fsm</> or <literal>_vm</> to find all the files associated
-with the relation.
+and/or <literal>_fsm</>, <literal>_vm</>, or <literal>_init</> to find all
+the files associated with the relation.
 </para>
 
 <para>