Standardize ItemIdData terminology.

The term "item pointer" should not be used to refer to ItemIdData
variables, since that is needlessly ambiguous.  Only
ItemPointerData/ItemPointer variables should be called item pointers.

To fix, establish the convention that ItemIdData variables should always
be referred to either as "item identifiers" or "line pointers".  The
term "item identifier" already predominates in docs and translatable
messages, and so should be the preferred alternative there.

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

src/include/access/htup_details.h

@@ -564,7 +564,7 @@ do { \
  * MaxHeapTuplesPerPage is an upper bound on the number of tuples that can
  * fit on one heap page.  (Note that indexes could have more, because they
  * use a smaller tuple header.)  We arrive at the divisor because each tuple
- * must be maxaligned, and it must have an associated item pointer.
+ * must be maxaligned, and it must have an associated line pointer.
  *
  * Note: with HOT, there could theoretically be more line pointers (not actual
  * tuples) than this on a heap page.  However we constrain the number of line

src/include/access/itup.h

@@ -131,7 +131,7 @@ typedef IndexAttributeBitMapData * IndexAttributeBitMap;
  * fit on one index page.  An index tuple must have either data or a null
  * bitmap, so we can safely assume it's at least 1 byte bigger than a bare
  * IndexTupleData struct.  We arrive at the divisor because each tuple
- * must be maxaligned, and it must have an associated item pointer.
+ * must be maxaligned, and it must have an associated line pointer.
  *
  * To be index-type-independent, this does not account for any special space
  * on the page, and is thus conservative.