Document functions behavior w.r.t. "Skip this version"

Document that both win_sparkle_check_update_without_ui() and
win_sparkle_check_update_with_ui() ignore the "Skip this version"
setting.

See #64.

include/winsparkle.h

@@ -293,6 +293,10 @@ WIN_SPARKLE_API void __cdecl win_sparkle_set_shutdown_request_callback(win_spark
 
     This function returns immediately.
 
+    @note Because this function is intended for manual, user-initiated checks
+          for updates, it ignores "Skip this version" even if the user checked
+          it previously.
+
     @see win_sparkle_check_update_without_ui()
  */
 WIN_SPARKLE_API void __cdecl win_sparkle_check_update_with_ui();
@@ -309,6 +313,9 @@ WIN_SPARKLE_API void __cdecl win_sparkle_check_update_with_ui();
 
     This function returns immediately.
 
+    @note This function currently ignores "Skip this version" even if the user
+          checked it previously.
+
     @since 0.4
 
     @see win_sparkle_check_update_with_ui()