Clear up confusion w/ presence around belongs_to associations

[mcmire]

Now that belongs_to associations add a presence validation
automatically, if you have a test such as the following:

class StudentBook < ApplicationRecord
  belongs_to :student
end

RSpec.describe StudentBook do
  it { is_expected.not_to be_valid }
  it { is_expected.to validate_presence_of(:student) }
end

then the test for presence of :student will fail, because the validation
message on the automatic presence validation is different than the usual
message. The solution here, of course, is to just use belong_to, but
that is not very obvious. So this commit updates the presence matcher to
remind users about using belong_to in a case such as this.

---

Fixes #1095.