Add documentation for modbus_mask_write_register (closes #91)

doc/Makefile.am

@@ -14,6 +14,7 @@ MAN3 = \
         modbus_get_socket.3 \
         modbus_mapping_free.3 \
         modbus_mapping_new.3 \
+        modbus_mask_write_register.3 \
         modbus_new_rtu.3 \
         modbus_new_tcp_pi.3 \
         modbus_new_tcp.3 \

doc/modbus_mask_write_register.txt

@@ -0,0 +1,39 @@
+modbus_mask_write_register(3)
+=============================
+
+
+NAME
+----
+modbus_mask_write_register - mask a single register
+
+
+SYNOPSIS
+--------
+*int modbus_mask_write_register(modbus_t *'ctx', int 'addr', uint16_t 'and', uint16_t 'or');*
+
+
+DESCRIPTION
+-----------
+The _modbus_mask_write_register()_ function shall modify the value of the 
+holding register at the address 'addr' of the remote device using the algorithm:
+new value = (current value AND 'and') OR ('or' AND (NOT 'and'))
+
+The function uses the Modbus function code 0x16 (mask single register).
+
+
+RETURN VALUE
+------------
+The _modbus_mask_write_register()_ function shall return 1 if successful. Otherwise
+it shall return -1 and set errno.
+
+
+SEE ALSO
+--------
+linkmb:modbus_read_registers[3]
+linkmb:modbus_write_registers[3]
+
+
+AUTHORS
+-------
+The libmodbus documentation was written by Stéphane Raimbault
+<stephane.raimbault@gmail.com>