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/backend/access/index/indexam.c

@@ -38,32 +38,6 @@
  *		This file contains the index_ routines which used
  *		to be a scattered collection of stuff in access/genam.
  *
- *
- * old comments
- *		Scans are implemented as follows:
- *
- *		`0' represents an invalid item pointer.
- *		`-' represents an unknown item pointer.
- *		`X' represents a known item pointers.
- *		`+' represents known or invalid item pointers.
- *		`*' represents any item pointers.
- *
- *		State is represented by a triple of these symbols in the order of
- *		previous, current, next.  Note that the case of reverse scans works
- *		identically.
- *
- *				State	Result
- *		(1)		+ + -	+ 0 0			(if the next item pointer is invalid)
- *		(2)				+ X -			(otherwise)
- *		(3)		* 0 0	* 0 0			(no change)
- *		(4)		+ X 0	X 0 0			(shift)
- *		(5)		* + X	+ X -			(shift, add unknown)
- *
- *		All other states cannot occur.
- *
- *		Note: It would be possible to cache the status of the previous and
- *			  next item pointer using the flags.
- *
  *-------------------------------------------------------------------------
  */