[clang][docs] Update Include Options Help (llvm#101192)

This adds HelpTexts for some clang options that relate to include
directory handling, to sync them up with what's in clang.rst (and
therefore the man page).

Author: lenary

clang/docs/CommandGuide/clang.rst

@@ -703,6 +703,10 @@ Preprocessor Options
 
   Do not search clang's builtin directory for include files.
 
+.. option:: -nostdinc++
+
+  Do not search the system C++ standard library directory for include files.
+
 .. option:: -fkeep-system-includes
 
   Usable only with :option:`-E`. Do not copy the preprocessed content of

clang/include/clang/Driver/Options.td

@@ -5507,7 +5507,7 @@ def no__dead__strip__inits__and__terms : Flag<["-"], "no_dead_strip_inits_and_te
 def nobuiltininc : Flag<["-"], "nobuiltininc">,
   Visibility<[ClangOption, CC1Option, CLOption, DXCOption]>,
   Group<IncludePath_Group>,
-  HelpText<"Disable builtin #include directories">,
+  HelpText<"Disable builtin #include directories only">,
   MarshallingInfoNegativeFlag<HeaderSearchOpts<"UseBuiltinIncludes">>;
 def nogpuinc : Flag<["-"], "nogpuinc">, Group<IncludePath_Group>,
   HelpText<"Do not add include paths for CUDA/HIP and"
@@ -5534,8 +5534,10 @@ def noprofilelib : Flag<["-"], "noprofilelib">;
 def noseglinkedit : Flag<["-"], "noseglinkedit">;
 def nostartfiles : Flag<["-"], "nostartfiles">, Group<Link_Group>;
 def nostdinc : Flag<["-"], "nostdinc">,
-  Visibility<[ClangOption, CLOption, DXCOption]>, Group<IncludePath_Group>;
-def nostdlibinc : Flag<["-"], "nostdlibinc">, Group<IncludePath_Group>;
+  Visibility<[ClangOption, CLOption, DXCOption]>, Group<IncludePath_Group>,
+  HelpText<"Disable both standard system #include directories and builtin #include directores">;
+def nostdlibinc : Flag<["-"], "nostdlibinc">, Group<IncludePath_Group>,
+  HelpText<"Disable standard system #include directories only">;
 def nostdincxx : Flag<["-"], "nostdinc++">, Visibility<[ClangOption, CC1Option]>,
   Group<IncludePath_Group>,
   HelpText<"Disable standard #include directories for the C++ standard library">,