docs(storage/fatfs): discrepancies between FATFS and POSIX

Closes #15187

Author: haberturdeur

docs/en/api-reference/storage/fatfs.rst

@@ -43,10 +43,11 @@ Most applications use the following workflow when working with ``esp_vfs_fat_``
 
 The convenience functions :cpp:func:`esp_vfs_fat_sdmmc_mount`, :cpp:func:`esp_vfs_fat_sdspi_mount`, and :cpp:func:`esp_vfs_fat_sdcard_unmount` wrap the steps described above and also handle SD card initialization. These functions are described in the next section.
 
-.. note::
-
-   Because FAT filesystem does not support hardlinks, :cpp:func:`link` copies contents of the file instead. (This only applies to files on FatFs volumes.)
+Differences to POSIX standard
+-----------------------------
 
+#. :cpp:func:`link`: Because FAT filesystem does not support hardlinks, :cpp:func:`link` copies contents of the file instead. (This only applies to files on FatFs volumes.)
+#. :cpp:func:`unlink`: When trying to remove an open file, the operation will either fail with ``EBUSY``, when ``CONFIG_FATFS_FS_LOCK`` is enabled, or the behaviour is undefined (possibly causing FS corruption), if not.
 
 .. _using-fatfs-with-vfs-and-sdcards: