doc: fs: document preconditions with fs_open & fs_opendir

fs_open and fs_opendir rely on the structures in the first param
being intialized. (The init functions are documented with the type
declarations.) This simple documentation change mentions this need
to initialize (and emphasizes it). (Users of similar APIs such as
POSIX open and opendir do not expect this initialization need.)

Fixes zephyrproject-rtos#97205
Signed-off-by: Paul Hepworth <[email protected]>

Author: phepworth

include/zephyr/fs/fs.h

@@ -297,7 +297,7 @@ static inline void fs_dir_t_init(struct fs_dir_t *zdp)
  * @warning If @p flags are set to 0 the function will open file, if it exists
  *          and is accessible, but you will have no read/write access to it.
  *
- * @param zfp Pointer to a file object
+ * @param zfp Pointer to an @b initialized file object
  * @param file_name The name of a file to open
  * @param flags The mode flags
  *
@@ -506,7 +506,7 @@ int fs_mkdir(const char *path);
  *
  * Opens an existing directory specified by the path.
  *
- * @param zdp Pointer to the directory object
+ * @param zdp Pointer to the @b initialized directory object
  * @param path Path to the directory to open
  *
  * @retval 0 on success;