[docs] Indicate which fields are optional in the model

### What happened?

The documentation does not indicate which fields are optional and which are required: https://allurereport.org/docs/how-it-works-test-result-file/

to find this information, I had to go to the [js source code](https://github.com/allure-framework/allure3/blob/main/packages/core-api/src/model.ts)
This information is not even represented documented in some of the modules, like the java module, though that is an issue for the respective modules.

### How can we reproduce it?

N/A

### What did you expect?

which fields are optional and which are required to be documented.

### Environment

N/A

### Additional context

_No response_

Start with the test-result-file documentation page and compare its model fields with packages/core-api/src/model.ts, which the issue identifies as the source of the optionality information. Check the relevant module documentation, including the Java module context, and update the field descriptions so required and optional fields are distinguishable. Done means the documentation explicitly records this information for the documented model.

Written by the indexing model from the issue text.