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.