Although there may be way to do this, I suggest you think about why you need this, since there are other checks in place that may make such a restriction unnecessary.
For example, if you are using standard locking on work objects, even if you allow the same user to log in more than one requestor, only one of the requestors will be allowed to open any given assignment. If this standard restriction is sufficient, you needn't worry about the multiple logins.
I don't have any document as such. I can provide you detailed steps with screenshots but they might not be useful for you as due to the bug in 7.1.8 you'll end up preventing all the logins to the system rendering the system useless until you change the system name back and restart the system
Please follow the below steps for stopping concurrent sessions in a single node:
Go to instances of record type system(records explorer>sysadmin>System).
Open any existing system name and save as that into any name you desire.(let it be abc)
Open clipboard for that newly created rule and go to page pySaveAsResults of class Data-Admin-System.
Now change the pyConcurrentSessions value to none from unlimited.
Save the clipboard and save the rule form.
Now we have to create "requestor types" for this newly created system name.
We have to create 4 requestor types namely App, Batch, Portal, Browser.
For this we can save as the existing requestor types to point to new system name.
After creating all the requestor types we need to change the prconfig.xml to point to new system name. This you can do by the following statement: <env name="Identification/SystemName" value="abc"/>
NOTE: Please check if the requestor types are created properly. Without the requestor types properly created you cannot access the system. You can see all the requestor types from System-General Landing page(Designer studio>System>general>Systems,nodes and requestors).
Please revert back if you are struck at some point.