[diagtool] explain that yellow is used to denote disabled-by-default warnings

[nickdesaulniers]

Summary

This is already implied, but let's just be explicit about it.

[nickdesaulniers]

clang/tools/diagtool/TreeView.cpp

130–131

it was clang-format, I swear!

clang/tools/diagtool/TreeView.cpp

int showAll() {
     }
 
     return 0;
   }
 
   void showKey() {
     out << '\n' << Colors::GREEN << "GREEN" << Colors::RESET
         << " = enabled by default";
+    out << '\n'
+        << Colors::YELLOW << "YELLOW" << Colors::RESET
+        << " = disabled by default";
     out << '\n' << Colors::RED << "RED" << Colors::RESET
         << " = unimplemented (accepted for GCC compatibility)\n\n";
   }
 };
 
 static void printUsage() {
   llvm::errs() << "Usage: diagtool tree [--internal] [<diagnostic-group>]\n";
 }