std docs list all NEON intrinsics as unstable

[tmandry]

std docs are assumed to be the source of truth but claim that all neon intrinsics (including the recently stabilized ones on arm64) are still unstable, which is very confusing*.

From #90972 (comment) and #90972 (comment):

The intrinsics that are shared with the 32-bit ARM targets are basically partially stable; stable on AArch64, unstable on 32-bit. Rustdoc doesn't seem to be able to handle that.

Even though these intrinsics are exposed behind a aarch64 module, they are internally shared with other arm platforms, hence the need to have target-dependent stability attributes on them.

(*) Source: I know multiple people who were confused, myself included!

cc #90972

[ehuss]

This is tracked on the stdarch side as rust-lang/stdarch#1268, which has some more detail on the situation.

[Dylan-DPC]

Closing this as it is tracked by the issue linked above.