docs: silence remaining @doc false cross-reference warnings in mix docs

`mix docs` was emitting these warnings:

  warning: documentation references function
           "Longbridge.OAuth.token_path/1" but it is hidden
  warning: documentation references function
           "Longbridge.Connection.Session.broadcast/2" but it is hidden
  warning: documentation references function
           "Longbridge.Connection.Session.cancel_idle_timer/1"
           but it is hidden
  ... 7 more Session.<fn> references in lib/longbridge/ws_connection.ex
  warning: documentation references module
           "Longbridge.Application" but it is hidden

All 11 references are to @doc false functions/modules:
  - Longbridge.OAuth.token_path/1: a back-compat defdelegate kept
    for callers that imported it before the public function moved
    to FileTokenStorage.
  - Longbridge.Connection.Session: an internal GenServer-style
    state machine. The defdelegates in WSConnection are for code
    clarity (call sites read like Longbridge.WSConnection.<fn>
    instead of Longbridge.Connection.Session.<fn>).
  - Longbridge.Application: the standard mix.exs Application
    callback module. Not part of the public API.

Extend skip_undefined_reference_warnings_on in mix.exs to include
the three offending files. The cross-references are intentional
and accurate — the warning is just because ex_doc refuses to
follow @doc false identifiers.

Verified: `mix docs` now runs clean with no warnings.

Author: gilbertwong96

mix.exs

@@ -42,7 +42,19 @@ defmodule Longbridge.MixProject do
       extras: ["README.md"],
       skip_undefined_reference_warnings_on: [
         "README.md",
-        "lib/longbridge/quote_context.ex"
+        "lib/longbridge/quote_context.ex",
+        # References `Longbridge.OAuth.token_path/1` which is @doc false
+        # (a back-compat shim kept for callers that imported it before
+        # the public function was moved to FileTokenStorage).
+        "lib/longbridge/oauth/file_token_storage.ex",
+        # References `Longbridge.Connection.Session` defdelegates which
+        # are @doc false (the Session module is internal-only; the
+        # defdelegate is for clarity in the WSConnection code).
+        "lib/longbridge/ws_connection.ex",
+        # References `Longbridge.Application` which is @doc false
+        # (Application modules are entrypoints, not part of the
+        # public API).
+        "lib/longbridge/symbol/store.ex"
       ]
     ]
   end