Add comment about rationalizing denominators

[siwelwerd]

While working on PF3, there are a few places I think it makes more sense to not rationalize denominators--for example, it is easier to see the reciprocal relationship between $\cos\left(\dfrac{\pi}{4}\right)=\dfrac{1}{\sqrt{2}}$ and $\sec\left(\dfrac{\pi}{4}\right)=\sqrt{2}$. We seem to also use non-rationalized expressions as early as TR4. @AbbyANoble noted we should be consistent, so I think we should add a remark or similar in TR4 or TR5 noting that some texts insist on rationalizing, but we don't. In general I don't really care if students write $\dfrac{\sqrt{2}}{2}$ or $\dfrac{1}{\sqrt{2}}$, but I do think it is important for them to be able to see either one written and recognize it as the sine/cosine of $\dfrac{\pi}{4}$.

Feel free to make the case here that we should insist on rationalizing.