I have created a trasition and a rule but I don't see any option whether in the property editor or somewhere else to link the transition with the rule that I have created.
It depends on what you are trying to do here. In composer there is an option to Restrict the transition based on a rule. is that what you are trying to do? If so, in composer, click on the transition, then open the property editor, and click on the Restrict by Rule. then you will get a dropdown list option to select the rule you want to use.
Restrict by Rule generally applies to regular transitions (not Update transitions) leaving a State, not a Decision. So in other words, you should be able to restrict by transition going into a decision. Then, use the rules assigned to branches from the transition to control the desired outcome - as stated, the Otherwise branch neither has a rule nor can be restricted by rule.
This transition is going out of a decision (the diamond "valid docs"). Because of this, it cannot have a rule. If you want to restrict this transition, you will need to edit the rules on the "valid docs" decision instead of adding the rule to the transition. If you need more clarification, please share a screen print of the properties for the Valid Docs decision. It will look something like this:
That will help us to explain better.