Add natspec to `Escrow` smart contract

## DEADLINE (including timezone)


## About feature 

https://docs.soliditylang.org/en/latest/natspec-format.html

## Describe the solution you'd like


## Describe alternatives you've considered

## Screenshots
 - Device: [e.g. iPhone6]
 - OS: [e.g. iOS8.1]
 - Browser [e.g. stock browser, safari]
 - Version [e.g. 22]

## Linked issues
#xx

## Additional context
