fix: Update the deprecation message to link to the docs (#6211)

The deprecation message is currently a bit confusing. Users may not
understand what is `[features].x`. I updated the docs and the
deprecation message for more guidance.

---------

Co-authored-by: Gabriel Peal <gpeal@users.noreply.github.com>

codex-rs/core/tests/suite/deprecation_notice.rs

@@ -43,7 +43,7 @@ async fn emits_deprecation_notice_for_legacy_feature_flag() -> anyhow::Result<()
     assert_eq!(
         details.as_deref(),
         Some(
-            "You can either enable it using the CLI with `--enable streamable_shell` or through the config.toml file with `[features].streamable_shell`"
+            "Enable it with `--enable streamable_shell` or `[features].streamable_shell` in config.toml. See https://github.com/openai/codex/blob/main/docs/config.md#feature-flags for details."
         ),
     );