Document macros for error codes corresponding to Modbus exceptions (#758

)

* Document macros for Modbus error codes
* Add exception response codes to docs

Thank you @kyllingstad

docs/index.md

@@ -246,6 +246,21 @@ shall return either a NULL value (if returning a pointer) or a negative value
 (if returning an integer), and the actual error code shall be stored in the
 `errno` variable.
 
+This includes cases where a Modbus server returns an exception response as a
+result of a failed read or write operation. The following macros expand to
+integer error codes that correspond to Modbus exception responses:
+
+- `EMBXILFUN`: illegal function (0x01)
+- `EMBXILADD`: illegal data address (0x02)
+- `EMBXILVAL`: illegal data value (0x03)
+- `EMBXSFAIL`: server device failure (0x04)
+- `EMBXACK`: acknowledge (0x05)
+- `EMBXSBUSY`: server device busy (0x06)
+- `EMBXNACK`: negative acknowledge (0x07)
+- `EMBXMEMPAR`: memory parity error (0x08)
+- `EMBXGPATH`: gateway path unavailable (0x0A)
+- `EMBXGTAR`: gateway target device failed to respond (0x0B)
+
 The *modbus_strerror()* function is provided to translate libmodbus-specific
 error codes into error message strings; for details refer to
 [modbus_strerror](modbus_strerror.md).