[compiler-rt] libFuzzer: update -merge_control_file= help message.

[Dor1s]

Summary

The motivation for this change is to have a distinguisher in libFuzzer
that would let the runner know whether multistep merge is supported or not by
a particular fuzz target binary. Otherwise, multistep merge fails to execute
with older version of libFuzzer, and there is no way to verify that easily.

[Dor1s]

Landed as https://github.com/llvm/llvm-project/commit/926fa4088cc2d6fdcd9301e80d05d9310009b660, not sure why it hasn't closed automatically.

compiler-rt/lib/fuzzer/FuzzerFlags.def

FUZZER_FLAG_INT(merge, 0, "If 1, the 2-nd, 3-rd, etc corpora will be "
   "merged into the 1-st corpus. Only interesting units will be taken. "
   "This flag can be used to minimize a corpus.")
 FUZZER_FLAG_STRING(stop_file, "Stop fuzzing ASAP if this file exists")
 FUZZER_FLAG_STRING(merge_inner, "internal flag")
 FUZZER_FLAG_STRING(merge_control_file,
                    "Specify a control file used for the merge process. "
                    "If a merge process gets killed it tries to leave this file "
                    "in a state suitable for resuming the merge. "
-                   "By default a temporary file will be used.")
+                   "By default a temporary file will be used."
+                   "The same file can be used for multistep merge process.")
 FUZZER_FLAG_INT(minimize_crash, 0, "If 1, minimizes the provided"
   " crash input. Use with -runs=N or -max_total_time=N to limit "
   "the number attempts."
   " Use with -exact_artifact_path to specify the output."
   " Combine with ASAN_OPTIONS=dedup_token_length=3 (or similar) to ensure that"
   " the minimized input triggers the same crash."
   )
 FUZZER_FLAG_INT(cleanse_crash, 0, "If 1, tries to cleanse the provided"