Add missing docstring to get_cache_path function (#809)

* Initial plan

* Add missing docstring to get_cache_path function

Co-authored-by: lucemia <432851+lucemia@users.noreply.github.com>

* Update docstring format for get_cache_path

Co-authored-by: lucemia <432851+lucemia@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: lucemia <432851+lucemia@users.noreply.github.com>

Author: Copilot

src/ffmpeg/common/cache.py

@@ -10,6 +10,18 @@
 
 
 def get_cache_path() -> Path:
+    """
+    Get the cache directory path.
+
+    Returns the cache directory path, creating it if it doesn't exist.
+    When running as a frozen application (e.g., PyInstaller), uses the
+    temporary extraction directory. Otherwise, uses the cache subdirectory
+    relative to this module.
+
+    Returns:
+        Path: The cache directory path
+
+    """
     if getattr(sys, "frozen", False):
         base_path = Path(getattr(sys, "_MEIPASS", ""))
     else: