build-unix: Update UniValue build conditions

doc/build-unix.md

@@ -46,7 +46,7 @@ Optional dependencies:
  qt          | GUI              | GUI toolkit (only needed when GUI enabled)
  protobuf    | Payments in GUI  | Data interchange format used for payment protocol (only needed when GUI enabled)
  libqrencode | QR codes in GUI  | Optional for generating QR codes (only needed when GUI enabled)
- univalue    | Utility          | JSON parsing and encoding (if missing, bundled version will be used)
+ univalue    | Utility          | JSON parsing and encoding (bundled version will be used unless --with-system-univalue passed to configure)
  libzmq3     | ZMQ notification | Optional, allows generating ZMQ notifications (requires ZMQ version >= 4.x)
 
 For the versions used in the release, see [release-process.md](release-process.md) under *Fetch and build inputs*.