rebuild-testbench.sh: explain BUILD_TYPE=native in usage()

Explain what happens when running the script without any argument.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

scripts/rebuild-testbench.sh

@@ -2,7 +2,7 @@
 # SPDX-License-Identifier: BSD-3-Clause
 # Copyright(c) 2020, Mohana Datta Yelugoti
 
-# fail on any errors
+# stop on most errors
 set -e
 
 # Defaults
@@ -15,6 +15,7 @@ print_usage()
  cat <<EOFUSAGE
 usage: $0 [-f] [-p <platform>]
  -p Build testbench binary for xt-run for selected platform, e.g. -p tgl
+ When omitted, perform a BUILD_TYPE=native, compile-only check.
  -f Build testbench with compiler provided by fuzzer
  (default path: $HOME/sof/work/AFL/afl-gcc)
  -j number of parallel make/ninja jobs. Defaults to /usr/bin/nproc.
@@ -139,7 +140,7 @@ main()
  done
 
  rebuild_testbench
-
+ printf '\n'
  testbench_usage
 }