bios_table: pldm_bios_table_string_entry_decode_string_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: I501d42a07463ff5a58793a5850f65ca8932bbf57
diff --git a/include/libpldm/bios_table.h b/include/libpldm/bios_table.h
index e1da24a..0ec3671 100644
--- a/include/libpldm/bios_table.h
+++ b/include/libpldm/bios_table.h
@@ -143,7 +143,10 @@
  *  @param[in] entry - Pointer to a bios string table entry
  *  @param[out] buffer - Pointer to a buffer to store the string
  *  @param[in] size - Size of the buffer to store the string
- *  @return pldm_completion_codes
+ *  @return PLDM_SUCCESS on success, PLDM_ERROR_INVALID_DATA if entry or buffer are NULL, or
+ *          PLDM_ERROR_INVALID_LENGTH if size is insufficient for the decoded string. An
+ *          appropriate value for size can be determined using the expression
+ *          `pldm_bios_table_string_entry_decode_string_length(entry) + 1`.
  */
 int pldm_bios_table_string_entry_decode_string_check(
 	const struct pldm_bios_string_table_entry *entry, char *buffer,