Document suffixes used by `sets.v`.

docs/style_guide.md

@@ -119,3 +119,4 @@ is used by clients.
 * SI: step-indexed bunched implication logic (of type `sbi`)
 * T: canonical structures for algebraic classes (for example ofeT for OFEs, cmraT for cameras)
 * Σ: global camera functor management (`gFunctors`; see [proof\_guide.md](./proof_guide.md))
+* U, D, DF: sets for the union and disjoint union cameras (see the [sets module](theories/algebra/sets.v))