Document policy on the use of `rustc_legacy_const_generics` in stdarch

[traviscross]

@RalfJung asked a reasonable question after we FCPed:

• Policy on the use of rustc_legacy_const_generics in stdarch rust#149654

He asked, "shouldn't we write this down somewhere?" Probably we should, yes. Especially when the policy is just in a random issue rather than in an RFC (e.g. RFC rust-lang/rfcs#3531).

The right place to write these down is probably here, in this repo, on our lang team website. This issue tracks doing that, at least for this policy.