[sancov] extending sancov --help documentation

[aizatsky]

Full --help output with this change: https://gist.github.com/anonymous/9b20cc6e13b53cf923bf286f0a39972b

[kcc]

LGTM

llvm/trunk/tools/sancov/sancov.cc

cl::values(
         clEnumValN(HtmlReportAction, "html-report",
                    "REMOVED. Use -symbolize & coverage-report-server.py."),
         clEnumValN(SymbolizeAction, "symbolize",
                    "Produces a symbolized JSON report from binary report."),
         clEnumValN(MergeAction, "merge", "Merges reports.")));
 
 static cl::list<std::string>
     ClInputFiles(cl::Positional, cl::OneOrMore,
-                 cl::desc("(<binary file>|<.sancov file>)..."));
+                 cl::desc("<action> <binary files...> <.sancov files...> "
+                          "<.symcov files...>"));
 
 static cl::opt<bool> ClDemangle("demangle", cl::init(true),
                                 cl::desc("Print demangled function name."));
 
 static cl::opt<bool>
     ClSkipDeadFiles("skip-dead-files", cl::init(true),
                     cl::desc("Do not list dead source files in reports."));
 
int main(int Argc, char **Argv) {
   sys::PrintStackTraceOnErrorSignal(Argv[0]);
   PrettyStackTraceProgram X(Argc, Argv);
   llvm_shutdown_obj Y; // Call llvm_shutdown() on exit.
 
   llvm::InitializeAllTargetInfos();
   llvm::InitializeAllTargetMCs();
   llvm::InitializeAllDisassemblers();
 
-  cl::ParseCommandLineOptions(Argc, Argv, "Sanitizer Coverage Processing Tool");
+  cl::ParseCommandLineOptions(Argc, Argv, 
+      "Sanitizer Coverage Processing Tool (sancov)\n\n"
+      "  This tool can extract various coverage-related information from: \n"
+      "  coverage-instrumented binary files, raw .sancov files and their "
+      "symbolized .symcov version.\n"
+      "  Depending on chosen action the tool expects different input files:\n"
+      "    -print-coverage-pcs     - coverage-instrumented binary files\n"
+      "    -print-coverage         - .sancov files\n"
+      "    <other actions>         - .sancov files & corresponding binary "
+      "files, .symcov files\n"
+      );
 
   // -print doesn't need object files.
   if (Action == PrintAction) {
     readAndPrintRawCoverage(ClInputFiles, outs());
     return 0;
   } else if (Action == PrintCovPointsAction) {
     // -print-coverage-points doesn't need coverage files.
     for (const std::string &ObjFile : ClInputFiles) {