[RISCV] Add a release note about tail folding being enabled. NFC

It's probably useful for users to know how to get the old scalar epilogue back if they need it.

Author: lukel97

llvm/docs/ReleaseNotes.md

@@ -109,6 +109,9 @@ Changes to the PowerPC Backend
 Changes to the RISC-V Backend
 -----------------------------
 
+* The loop vectorizer now performs tail folding by default on RISC-V, which
+  removes the need for a scalar epilogue loop. To restore the previous behaviour
+  use `-mllvm -prefer-predicate-over-epilogue=scalar-epilogue`.
 * `llvm-objdump` now has basic support for switching between disassembling code
   and data using mapping symbols such as `$x` and `$d`. Switching architectures
   using `$x` with an architecture string suffix is not yet supported.