include: zephyr: nvmem: Add retval doxygen to read/write functions

Document the return values for the read/write NVMEM functions.

Signed-off-by: Pieter De Gendt <[email protected]>

Author: pdgendt

include/zephyr/nvmem.h

@@ -316,7 +316,9 @@ struct nvmem_cell {
  *
  * @kconfig_dep{CONFIG_NVMEM}
  *
- * @return 0 on success, negative errno code on failure.
+ * @retval -EINVAL Invalid offset or length arguments.
+ * @retval -ENXIO  No runtime device API available.
+ * @return the result of the underlying device API call.
  */
 int nvmem_cell_read(const struct nvmem_cell *cell, void *data, off_t off, size_t len);
 
@@ -330,7 +332,10 @@ int nvmem_cell_read(const struct nvmem_cell *cell, void *data, off_t off, size_t
  *
  * @kconfig_dep{CONFIG_NVMEM}
  *
- * @return 0 on success, negative errno code on failure.
+ * @retval -EINVAL Invalid offset or length arguments.
+ * @retval -EROFS  Writing to a read-only NVMEM Cell.
+ * @retval -ENXIO  No runtime device API available.
+ * @return the result of the underlying device API call.
  */
 int nvmem_cell_write(const struct nvmem_cell *cell, const void *data, off_t off, size_t len);