[Docs] Clarify typed pointers support timeline

As there have been a couple of questions about this recently, this
gives a hard timeline on typed pointers support.

Given that we are about a month away from LLVM 16 branching, I think
we should retain best-effort typed pointer support in LLVM 16 even
if we get all tests migrated before that point.

Conversely, regardless of what the actual test migration state will
be at that point, I believe we should un-support typed pointers as
a matter of policy immediately after branching. Once release/16.x
has been branched, typed pointers on main will no longer be
supported (and can be actively broken). We only need to keep
not-yet-migrated tests working, if there are any left at that point.

Differential Revision: https://reviews.llvm.org/D140487

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