PLCgen: Improve S7 TIA Portal usage documentation for TIA portal v15

Using the S7 output documentation page we loaded generated PLC-code onto a S7-300 PLC using TIA portal v15. The PLC-code was generated from CIF-models using the PLC-code generator. We found some differences between the documentation and our execution, see points below.

• At "Import the generated PLC code (PLC tags)": It is noted as a stap to import CONSTS.xml, which was not generated for us. We did have an Outputs.xml file which we needed to import.
• At "Configure the inputs and outputs": The IO-addresses don't need to be manually added. For generation of the PLC-code we already had to submit the IO-addresses and these get imported with the IO-table.
• At "Configure the inputs and outputs": Here it is mentioned to create outputs. This can be done earlier and in the same way as the inputs. Unless there is a reason we are unaware of to do this differently or later.
• At "Configure the inputs and outputs": The last step mentions opening "Main". It is a weird step, since it only opens a file but no further action is taken.
• At "Download PLC code to device": It may occur that there need to be certain actions taken when loading new code to the PLC. This depends on the state of the PLC (if it is running, or if it had a runtime error). Usually there is only one action to be taken, but you still have to do this manually before you can load. See below.
  

Addresses #679 Addresses #798 (closed)