Add a note about capturing screenshots for caught assertion errors

[miherlosev]

A customer sets the takeScreenshotOnFails option.

The screenshot will be captured even the error is caught.

import fs from 'fs';

fixture `Fixture`;

test('Test', async t => {
    if(!fs.existsSync('screenshots'))
        fs.mkdirSync('screenshots');
    
    try {
        await t.expect(false).ok();
    }
    catch (e) {
    }

    await t.expect(fs.readdirSync('screenshots').length).eql(0);
});

See more information in #5157 issue.