[clang-format] Tweak help text a bit

[thakis]

Summary

In particular, make it clear that --style=file is the default,
since there's some confusion about this, e.g. here:
https://stackoverflow.com/questions/61455148/

[MyDeveloperDay]

LGTM

clang/lib/Format/Format.cpp

LangOptions getFormattingLangOpts(const FormatStyle &Style) {
   LangOpts.ObjC = 1;
   LangOpts.MicrosoftExt = 1;    // To get kw___try, kw___finally.
   LangOpts.DeclSpecKeyword = 1; // To get __declspec.
   LangOpts.C99 = 1; // To get kw_restrict for non-underscore-prefixed restrict.
   return LangOpts;
 }
 
 const char *StyleOptionHelpDescription =
-    "Coding style, currently supports:\n"
-    "  LLVM, GNU, Google, Chromium, Microsoft, Mozilla, WebKit.\n"
-    "Use -style=file to load style configuration from\n"
-    ".clang-format file located in one of the parent\n"
-    "directories of the source file (or current\n"
-    "directory for stdin).\n"
-    "Use -style=file:<format_file_path> to explicitly specify\n"
-    "the configuration file.\n"
-    "Use -style=\"{key: value, ...}\" to set specific\n"
-    "parameters, e.g.:\n"
-    "  -style=\"{BasedOnStyle: llvm, IndentWidth: 8}\"";
+    "Set coding style. <string> can be:\n"
+    "1. A preset: LLVM, GNU, Google, Chromium, Microsoft,\n"
+    "   Mozilla, WebKit.\n"
+    "2. 'file' to load style configuration from a\n"
+    "   .clang-format file in one of the parent directories\n"
+    "   of the source file (for stdin, see --assume-filename).\n"
+    "   If no .clang-format file is found, falls back to\n"
+    "   --fallback-style.\n"
+    "   --style=file is the default.\n"
+    "3. 'file:<format_file_path>' to explicitly specify\n"
+    "   the configuration file.\n"
+    "4. \"{key: value, ...}\" to set specific parameters, e.g.:\n"
+    "   --style=\"{BasedOnStyle: llvm, IndentWidth: 8}\"";
 
 static FormatStyle::LanguageKind getLanguageByFileName(StringRef FileName) {
   if (FileName.endswith(".java"))
     return FormatStyle::LK_Java;
   if (FileName.endswith_insensitive(".js") ||
       FileName.endswith_insensitive(".mjs") ||
       FileName.endswith_insensitive(".ts")) {
     return FormatStyle::LK_JavaScript; // (module) JavaScript or TypeScript.
if (Extension.empty() || Extension == ".h") {
       Guesser.process();
       if (Guesser.isObjC())
         return FormatStyle::LK_ObjC;
     }
   }
   return GuessedLanguage;
 }
 
+// Update StyleOptionHelpDescription above when changing this.
 const char *DefaultFormatStyle = "file";
 
 const char *DefaultFallbackStyle = "LLVM";
 
 llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>>
 loadAndParseConfigFile(StringRef ConfigFile, llvm::vfs::FileSystem *FS,
                        FormatStyle *Style, bool AllowUnknownOptions) {
   llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> Text =

clang/tools/clang-format/ClangFormat.cpp

static cl::opt<std::string>
     Style("style", cl::desc(clang::format::StyleOptionHelpDescription),
           cl::init(clang::format::DefaultFormatStyle),
           cl::cat(ClangFormatCategory));
 static cl::opt<std::string>
     FallbackStyle("fallback-style",
                   cl::desc("The name of the predefined style used as a\n"
                            "fallback in case clang-format is invoked with\n"
                            "-style=file, but can not find the .clang-format\n"
-                           "file to use.\n"
+                           "file to use. Defaults to 'LLVM'.\n"
                            "Use -fallback-style=none to skip formatting."),
                   cl::init(clang::format::DefaultFallbackStyle),
                   cl::cat(ClangFormatCategory));
 
 static cl::opt<std::string> AssumeFileName(
     "assume-filename",
-    cl::desc("Override filename used to determine the language.\n"
-             "When reading from stdin, clang-format assumes this\n"
-             "filename to determine the language.\n"
+    cl::desc("Set filename used to determine the language and to find\n"
+             ".clang-format file.\n"
+             "Only used when reading from stdin.\n"
+             "If this is not passed, the .clang-format file is searched\n"
+             "relative to the current working directory when reading stdin.\n"
              "Unrecognized filenames are treated as C++.\n"
              "supported:\n"
              "  CSharp: .cs\n"
              "  Java: .java\n"
              "  JavaScript: .mjs .js .ts\n"
              "  Json: .json\n"
              "  Objective-C: .m .mm\n"
              "  Proto: .proto .protodevel\n"