Removal of 'loop' Parameter in web.run_app

### Is your feature request related to a problem?

Yes, the absence of a detailed description for the 'loop' parameter in the web.run_app function documentation is causing confusion among developers. Users are unclear about the purpose and usage of this parameter, which can lead to improper implementations or avoidable errors.

### Describe the solution you'd like

I propose to add a comprehensive description of the 'loop' parameter including its purpose. This addition will provide clarity and assist developers in correctly utilizing the web.run_app function.

### Describe alternatives you've considered

An alternative would be to include links to relevant sections of the documentation where the 'loop' concept is explained in detail. However, directly adding the description within the web.run_app section ensures that the information is immediately accessible where it's most needed.

### Related component

Server

### Additional context

_No response_

### Code of Conduct

- [X] I agree to follow the aio-libs Code of Conduct