llvm-profdata: Clarify the top level help

It wasn't very obvious that you're supposed to call help on the
subcommands. This should help.

llvm-svn: 277678
diff --git a/llvm/tools/llvm-profdata/llvm-profdata.cpp b/llvm/tools/llvm-profdata/llvm-profdata.cpp
index aec689b..d5c05da 100644
--- a/llvm/tools/llvm-profdata/llvm-profdata.cpp
+++ b/llvm/tools/llvm-profdata/llvm-profdata.cpp
@@ -665,6 +665,7 @@
       errs() << "OVERVIEW: LLVM profile data tools\n\n"
              << "USAGE: " << ProgName << " <command> [args...]\n"
              << "USAGE: " << ProgName << " <command> -help\n\n"
+             << "See each individual command --help for more details.\n"
              << "Available commands: merge, show\n";
       return 0;
     }