Use has_css? instead of has_selector? in RSpec

While has_selector? supports CSS selectors and XPath selectors, has_css?
is most often how we assert elements are present in the DOM;
as an added benefit, it's shorter to type.
Reword to be more clear about matchers and Capybara
Favor "Prefer" over "Use" in this case
Fix wording for "Prefer"
Fix botched edit