bios_table: pldm_bios_table_attr_entry_string_decode_def_string_length_check() values

Document the pre-conditions that lead to failures and their associated
error codes.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: Icc9301d25c83de39f613fc835c12e0565f5b5c8b
diff --git a/include/libpldm/bios_table.h b/include/libpldm/bios_table.h
index 533a480..4fe45dd 100644
--- a/include/libpldm/bios_table.h
+++ b/include/libpldm/bios_table.h
@@ -399,7 +399,8 @@
  * validity of the parameters
  *  @param[in] entry - Pointer to bios attribute table entry
  *  @param[out] def_string_length Pointer to length of default string in bytes
- *  @return pldm_completion_codes
+ *  @return PLDM_SUCCESS on success, otherwise PLDM_ERROR_INVALID_DATA if entry or def_string_length
+ *          are NULL, or entry is not of type PLDM_BIOS_STRING
  */
 int pldm_bios_table_attr_entry_string_decode_def_string_length_check(
 	const struct pldm_bios_attr_table_entry *entry,