My LessWrong link is to the highlighted text, because (as far as I'm aware) it's currently not possible to link to a specific section of a LessWrong tag/entry/concept. This seems a bit weird and messy to me, but still better than the alternatives (don't include that link at all or just link to the page as a whole).
These terms may basically only be used on the LessWrong community, and may not be prominent or useful enough to warrant an entry here. Not sure.
But Pablo said "I think this [entry] would be useful to have", and I lean towards the same view.
The distinction is commonly used in academic philosophy, from which LW probably took it. On reflection, it may be worth having separate articles on instrumental and epistemic rationality, though I guess the two notions can also be discussed in the same article.
I think someone should be able to quickly add useful text here by drawing on the LessWrong link I put in Further reading and the relevant section of the Morality vs. related concepts post.