[clang-doc] Add missing comment for Base (llvm#138053)

Author: ilovepi

clang-tools-extra/clang-doc/Representation.h

@@ -535,6 +535,7 @@ struct ClangDocContext {
   std::vector<std::string> UserStylesheets;
   // JavaScript files that will be imported in all HTML files.
   std::vector<std::string> JsScripts;
+  // Base directory for remote repositories.
   StringRef Base;
   Index Idx;
 };