[clang-format] update --files help description

correlates the option with reponse file concept.

Reviewed By: HazardyKnusperkeks

Differential Revision: https://reviews.llvm.org/D135115

NOKEYCHECK=True
GitOrigin-RevId: 2c94f75f00af16b6b4c2336e5cbfd8187d8bf33c
diff --git a/ClangFormat.cpp b/ClangFormat.cpp
index 269bc59..9e8b881 100644
--- a/ClangFormat.cpp
+++ b/ClangFormat.cpp
@@ -125,9 +125,11 @@
              "determined by the QualifierAlignment style flag"),
     cl::init(""), cl::cat(ClangFormatCategory));
 
-static cl::opt<std::string>
-    Files("files", cl::desc("Provide a list of files to run clang-format"),
-          cl::init(""), cl::cat(ClangFormatCategory));
+static cl::opt<std::string> Files(
+    "files",
+    cl::desc("A file containing a list of files to process, one per line."),
+    cl::value_desc("filename"),
+    cl::init(""), cl::cat(ClangFormatCategory));
 
 static cl::opt<bool>
     Verbose("verbose", cl::desc("If set, shows the list of processed files"),