Update the file_write description in fs.h

Replace with the description taken from fs_write.c

Signed-off-by: junmin-kim <junmindd.kim@samsung.com>

include/nuttx/fs/fs.h

@@ -1073,8 +1073,24 @@ ssize_t nx_read(int fd, FAR void *buf, size_t nbytes);
  *
  * Description:
  *   Equivalent to the standard write() function except that is accepts a
- *   struct file instance instead of a file descriptor.  Currently used
- *   only by aio_write();
+ *   struct file instance instead of a file descriptor.  It is functionally
+ *   equivalent to write() except that in addition to the differences in
+ *   input paramters:
+ *
+ *  - It does not modify the errno variable,
+ *  - It is not a cancellation point, and
+ *  - It does not handle socket descriptors.
+ *
+ * Input Parameters:
+ *   filep  - Instance of struct file to use with the write
+ *   buf    - Data to write
+ *   nbytes - Length of data to write
+ *
+ * Returned Value:
+ *  On success, the number of bytes written are returned (zero indicates
+ *  nothing was written).  On any failure, a negated errno value is returned
+ *  (see comments withwrite() for a description of the appropriate errno
+ *  values).
  *
  ****************************************************************************/