Document PointerLike implementation restrictions.

[kpreid]

Since #133226, it is no longer automatically implemented, but must be manually implemented, with special restrictions. The documentation now (roughly) explains those special restrictions.

cc @compiler-errors (author of the previous PR)

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[kpreid]

Whoops, looks like someone got to this already today, in 159dba8.
I still have something to add, though. Resolved the conflict.

[tgross35]

This seems consistent with current use and with intent.

@bors r+ rollup

[bors]

📌 Commit da6616c has been approved by tgross35

It is now in the queue for this repository.