Auto merge of #10868 - Muscraft:add-reason-for-nightly-tests, r=ehuss

add a reason to `masquerade_as_nightly_cargo` so it is searchable

When I was working on the stabilization for workspace inheritance, it was very tedious to find all of the places to remove `.masquerade_as_nightly_cargo()`. I [suggested](https://rust-lang.zulipchat.com/#narrow/stream/246057-t-cargo/topic/problems.20finding.20.60.2Emasquerade_as_nightly_cargo.28.29.60) to add a reason to `.masquerade_as_nightly_cargo()` so that it would be easier to find all of the places to remove it. By adding the reason it makes it easy to search for all places with the features name. This PR adds the reason(s) to all of the places `.masquerade_as_nightly_cargo()` is called, as well as updates the documentation so it talks about adding a reason when making the call.