message: Explain initial value for exclusive scans
diff: @@ -4191,6 +4191,10 @@ bitwise-or are available:
  int64 exclusive_scan_or(int64 v)
  unsigned int64 exclusive_scan_or(unsigned int64 v)
  +The returned value for the first program instance will be ``0`` for
  +``exclusive_scan_add`` and ``exclusive_scan_or``, and have all bits set to
  +``1`` for ``exclusive_scan_and``.
  +
  The use of exclusive scan to generate variable amounts of output from
  program instances into a compact output buffer is `discussed in the FAQ`_.
  