fs: Clarify description of fs_open when no access bits given

Add warning that file opened without R/W flags will have no read/write
access.
Remove suggestion for using fs_open to check if file exists.
Clarify -ENOENT return reason.

Fixes #64030

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
diff --git a/subsys/fs/fs.c b/subsys/fs/fs.c
index bf3cd71..e959f34 100644
--- a/subsys/fs/fs.c
+++ b/subsys/fs/fs.c
@@ -1,7 +1,7 @@
 /*
  * Copyright (c) 2018 Intel Corporation.
  * Copyright (c) 2020 Peter Bigot Consulting, LLC
- * Copyright (c) 2020 Nordic Semiconductor ASA
+ * Copyright (c) 2020-2024 Nordic Semiconductor ASA
  *
  * SPDX-License-Identifier: Apache-2.0
  */