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/cli/src/mcp_cmd.rs

@@ -353,7 +353,9 @@ async fn run_login(config_overrides: &CliConfigOverrides, login_args: LoginArgs)
         .context("failed to load configuration")?;
 
     if !config.features.enabled(Feature::RmcpClient) {
-        bail!("OAuth login is only supported when [features].rmcp_client is true in config.toml.");
+        bail!(
+            "OAuth login is only supported when [features].rmcp_client is true in config.toml. See https://github.com/openai/codex/blob/main/docs/config.md#feature-flags for details."
+        );
     }
 
     let LoginArgs { name, scopes } = login_args;

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 {

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."
         ),
     );