[NFC] Add more reduction docs for Two (#8132)

Author: kripken

scripts/fuzz_opt.py

@@ -1819,7 +1819,18 @@ def ensure(self):
 # run) with "second.wasm" as needed.
 #
 # In both cases, make sure to copy the files to a saved location first (do not
-# use a path to the scratch files that get constantly overwritten).
+# use a path to the scratch files that get constantly overwritten). The full
+# process might look like this:
+#
+#  * cp out/test/original.wasm first.wasm
+#  * cp out/test/second.wasm second.wasm
+#  * BINARYEN_FIRST_WASM=`pwd`/first.wasm [copied reducer command, replacing
+#                                          original.wasm with `pwd`/second.wasm]
+#  * cp out/test/w.wasm second.reduced.wasm
+#  * BINARYEN_SECOND_WASM=`pwd`/second.reduced.wasm [copied reducer command,
+#                                                    replacing original.wasm
+#                                                    with `pwd`/first.wasm]
+#
 class Two(TestCaseHandler):
     # Run at relatively high priority, as this is the main place we check cross-
     # module interactions.