Correct KeyNodeSynthesiser opt-out javadoc: @key(resolvable: false) does not keep a type out of _Entity

KeyNodeSynthesiser’s class javadoc carries an "Opt-out" paragraph claiming that a consumer who writes `@key(fields: "id", resolvable: false) on a @node type "keeps it out of _Entity`". That is false for the pinned federation-jvm version: `Federation.transform injects every @key-bearing type into the _Entity union regardless of resolvable:. The behaviour is empirically pinned by FederationBuildSmokeTest.resultEntityUnionContainsAllFixtureEntities, which asserts both the table-bound Language stub and (since R286) the non-table-bound FilmRefStub, both @key(resolvable: false), are present in the served _Entity union. resolvable: false does not suppress union membership; what it does is tell the supergraph composer not to route entity-resolution queries to this subgraph for that type. This is the "broader failure mode" the Documentation names only live tests/code principle warns about: a doc claim that a live test directly contradicts, surfaced during the R286 (12a9f88 + 77362d3) In Review → Done review. Surface flagged, not introduced by R286 (the test’s prior javadoc already noted "federation still includes them in the union"). Fix: rewrite the opt-out paragraph to describe what resolvable: false actually does (composer routing, not union membership), and audit KeyNodeSynthesiser:22 ("surfaces them in `_Entity`") for the same precision. No code change; doc-only, but verify no other javadoc/spec prose repeats the false claim.