doc: FIFO API uses first 32 bits of data items

API documenation for Nanokernel FIFO make no mention that the data items
added to the FIFO must include and reserve the first 32 bits for use as
a pointer to the next item in the FIFO.  This adds that note, as described
in the overview FIFO documentation.

Change-Id: I8cf0072d97dbb373a73d534f39b72831c726f4ef
Jira: ZEP-747
Signed-off-by: David B. Kinder <david.b.kinder@intel.com>

include/nanokernel.h

@@ -505,6 +505,11 @@ extern void nano_fifo_init(struct nano_fifo *fifo);
  * it should be avoided when the context is known up-front to avoid unnecessary
  * overhead.
  *
+ * FIFO data items must be aligned on a 4-byte boundary, as the kernel reserves
+ * the first 32 bits of each item for use as a pointer to the next data item in
+ * the FIFO's link list.  Each data item added to the FIFO must include and
+ * reserve these first 32 bits.
+ *
  * @param fifo FIFO on which to interact.
  * @param data Data to send.
  *