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

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

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D146827

clang/tools/diagtool/TreeView.cpp

@@ -127,6 +127,9 @@ public:
   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";
   }