Document issue with heapam line pointer truncation.

Document issue with heapam line pointer truncation.

Checking that an offset number isn't past the end of a heap page's line
pointer array was just a defensive sanity check for HOT-chain traversal
code before commit 3c3b8a4b.  It's etrictly necessary now, though.  Add
comments that reference the issue to code in heapam that needs to get it
right.

Per suggestion from Alexander Lakhin.

Discussion: https://postgr.es/m/f76a292c-9170-1aef-91a0-59d9443b99a3@gmail.com