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/src/codex.rs

@@ -553,7 +553,7 @@ impl Session {
                 None
             } else {
                 Some(format!(
-                    "You can either enable it using the CLI with `--enable {canonical}` or through the config.toml file with `[features].{canonical}`"
+                    "Enable it with `--enable {canonical}` or `[features].{canonical}` in config.toml. See https://github.com/openai/codex/blob/main/docs/config.md#feature-flags for details."
                 ))
             };
             post_session_configured_events.push(Event {