storage/stream_flash: Document write flush as mandatory

There is no need to make this optional just inform user that
this has to be called to complete write of buffers.
The commit also adds info that there should be no more write
attempts done with use of "flushed", as it may return write
errors anyway.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>

include/zephyr/storage/stream_flash.h

@@ -101,18 +101,20 @@ size_t stream_flash_bytes_written(struct stream_flash_ctx *ctx);
  * @brief Process input buffers to be written to flash device in single blocks.
  * Will store remainder between calls.
  *
- * A final call to this function with flush set to true
- * will write out the remaining block buffer to flash.
+ * A write with the @p flush set to true has to be issued as the last
+ * write request for a given context, as it concludes write of a stream,
+ * and flushes buffers to storage device.
+ *
+ * @warning There must not be any additional write requests issued for a flushed context,
+ * unless it is re-initialized, as such write attempts may result in the function
+ * failing and returning error.
+ * Once context has been flushed, it can be re-initialized and re-used for new
+ * stream flash session.
  *
  * @param ctx context
  * @param data data to write
  * @param len Number of bytes to write
  * @param flush when true this forces any buffered data to be written to flash
- *        A write with the @p flush set to true has to be issued as the last
- *        write request for a given context, as it concludes write of a stream;
- *        there must not be issued any more write requests for given context,
- *        unless it is re-initialized, and such write attempts may result in the
- *        function returning error.
  *
  * @return non-negative on success, negative errno code on fail
  */