docs(csharp): api-docs note says gen warns only when --generators names it

The registry Note shown by gen --list and the matching manifest note still said a
gen run that lists api-docs warns. After the gen change, an owned project that
lists it in codegen/Program.cs is silent; only a --generators selection naming it
warns. Text only.

Author: dmealing

fixtures/generator-registry-conformance/registry.json

@@ -90,7 +90,7 @@
       "concept": "Per-entity/template SDK API reference (generated-code API; human + agent forms).",
       "tier": "native",
       "layer": "docs",
-      "note": "Run by `meta docs` as its api surface (ADR-0025), not by `meta gen`: a `meta gen` config that lists it is warned about and skipped. On csharp it is run by `dotnet meta docs`, and `dotnet meta gen` skips it with a warning.",
+      "note": "Run by `meta docs` as its api surface (ADR-0025), not by `meta gen`: a `meta gen` config that lists it is warned about and skipped. On csharp it is run by `dotnet meta docs`, and `dotnet meta gen` skips it, warning only when `--generators` names it.",
       "ports": ["typescript", "csharp"]
     },
     "trace-helper": {

server/csharp/MetaObjects.Codegen/GeneratorRegistry.cs

@@ -262,7 +262,7 @@ private static IGenerator RenderHelper(GeneratorBuildContext ctx) =>
                 Tier = GeneratorTier.Native,
                 Layer = GeneratorLayer.Docs,
                 Factory = _ => new ApiDocsGenerator(),
-                Note = "Run by `dotnet meta docs` as its api surface (ADR-0025), not by `dotnet meta gen`: a `gen` run that lists it skips it with a warning.",
+                Note = "Run by `dotnet meta docs` as its api surface (ADR-0025), not by `dotnet meta gen`: `gen` skips it, and warns when `--generators` names it.",
                 SourceFileName = "ApiDocsGenerator.cs",
                 // The page IR, its builder, the renderer and the path math; and the field walk and the
                 // M:N navigation the builder documents with.