[SYCLomatic] Update help msg of option --rule-file (#2612)

Signed-off-by: Jiang, Zhiwei <zhiwei.jiang@intel.com>
Co-authored-by: Wang, Zhiming <zhiming.wang@intel.com>

Author: zhiweij1

clang/include/clang/DPCT/DPCTOptions.inc

@@ -435,8 +435,10 @@ DPCT_OPTION(clang::dpct::list, std::string, RuleFile,
                                 clang::dpct::DpctActionKind::DAK_BuildScript,
                                 clang::dpct::DpctActionKind::DAK_Analysis),
             "rule-file",
-            llvm::cl::desc("Specify the rule file path that "
-                           "contains rules used for migration.\n"),
+            llvm::cl::desc(
+                "Specify the rule file "
+                "for migration. Also, reference to the predefined rules in"
+                " \"extensions\" directory in the root folder of the tool.\n"),
             llvm::cl::value_desc("file"), llvm::cl::cat(CtHelpCatAdvanced),
             llvm::cl::ZeroOrMore)