Doc: Refer to line pointers as item identifiers.

An upcoming HEAD-only patch will standardize the terminology around
ItemIdData variables/line pointers, ending the practice of referring to
them as "item pointers".  Make the "Database Page Layout" docs
consistent with the new policy.  The term "item identifier" is already
used in the same section, so stick with that.

Discussion: https://postgr.es/m/CAH2-Wz=c=MZQjUzde3o9+2PLAPuHTpVZPPdYxN=E4ndQ2--8ew@mail.gmail.com
Backpatch: All supported branches.

doc/src/sgml/storage.sgml

@@ -759,14 +759,14 @@ free space pointers.</entry>
 
 <row>
 <entry>ItemIdData</entry>
-<entry>Array of (offset,length) pairs pointing to the actual items.
-4 bytes per item.</entry>
+<entry>Array of item identifiers pointing to the actual items. Each
+entry is an (offset,length) pair. 4 bytes per item.</entry>
 </row>
 
 <row>
 <entry>Free space</entry>
-<entry>The unallocated space. New item pointers are allocated from the start
-of this area, new items from the end.</entry>
+<entry>The unallocated space. New item identifiers are allocated from
+the start of this area, new items from the end.</entry>
 </row>
 
 <row>