Add rationale for `rustc_lint` behind allowing rustc::potential-query-instability lint

Context: https://github.com/rust-lang/rust/issues/84447

Motivation:

1. This lint asks to do so. An example:

   error: using `values` can result in unstable query results
      --> compiler/rustc_query_system/src/dep_graph/serialized.rs:654:50
       |
   654 |             let mut stats: Vec<_> = record_stats.values().collect();
       |                                                  ^^^^^^
       |
       = note: if you believe this case to be fine, allow this lint and add a comment explaining your rationale
   

2. This would prevent further fix attempts so wastes of time as well.
3. Nearly all of those allowances has a comment which explains the rationale.

To do: Add a comment explaning the rationale behind allowing rustc::potential-query-instability lint for this occurrence in the file compiler/rustc_lint/src/context/diagnostics/check_cfg.rs:

pub(super) fn unexpected_cfg_name(
    sess: &Session,
    (name, name_span): (Symbol, Span),
    value: Option<(Symbol, Span)>,
) -> lints::UnexpectedCfgName {
    #[allow(rustc::potential_query_instability)]
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    let possibilities: Vec<Symbol> = sess.psess.check_config.expecteds.keys().copied().collect();

    let mut names_possibilities: Vec<_> = if value.is_none() {

Open compiler/rustc_lint/src/context/diagnostics/check_cfg.rs and inspect unexpected_cfg_name, especially the allow(rustc::potential_query_instability) attribute. Compare nearby allowances and the linked context issue to understand the rationale style. Done means the allowance has an explanatory comment without changing the lint behavior.