[clangd] Mention the CLANGD_TRACE env var in the overview as well.

[sammccall]

That comment is talking about how CLANGD_FLAGS corresponds to command-line flags in general.

The CLANGD_TRACE environment variable controls only one piece of behavior, which doesn't correspond to any flag.
(It'd be reasonable to add a matching flag and describe the environment variable in that flag's description...)

clang-tools-extra/clangd/tool/ClangdMain.cpp

int main(int argc, char *argv[]) {
   const char *FlagsEnvVar = "CLANGD_FLAGS";
   const char *Overview =
       R"(clangd is a language server that provides IDE-like features to editors.
 
 It should be used via an editor plugin rather than invoked directly. For more information, see:
   https://clang.llvm.org/extra/clangd/
   https://microsoft.github.io/language-server-protocol/
 
-clangd accepts flags on the commandline, and in the CLANGD_FLAGS environment variable.
+clangd accepts flags on the commandline, and in the CLANGD_FLAGS/CLANGD_TRACE environment variable.
 )";
   llvm::cl::HideUnrelatedOptions(ClangdCategories);
   llvm::cl::ParseCommandLineOptions(argc, argv, Overview,
                                     /*Errs=*/nullptr, FlagsEnvVar);
   if (Test) {
     Sync = true;
     InputStyle = JSONStreamStyle::Delimited;
     LogLevel = Logger::Verbose;