document -isystem-cxx.

gnu/dist/gcc/gcc/doc/cpp.texi

@@ -1079,6 +1079,9 @@ command line.  If the same directory is named by both @option{-I} and
 @option{-isystem}, the @option{-I} option is ignored.  GCC provides an
 informative message when this occurs if @option{-v} is used.
 
+The @option{-isystem-cxx} command line option adds its argument to the
+list of C++ system headers, similar to @option{-isystem} for C headers.
+
 @findex #pragma GCC system_header
 There is also a directive, @code{@w{#pragma GCC system_header}}, which
 tells GCC to consider the rest of the current include file a system

gnu/dist/gcc/gcc/doc/cppopts.texi

@@ -448,6 +448,16 @@ is applied to the standard system directories.
 @xref{System Headers}.
 @end ifset
 
+@item -isystem-cxx @var{dir}
+@opindex isystemcxx
+Search @var{dir} for C++ header files, after all directories specified by
+@option{-I} but before the standard system directories.  Mark it
+as a system directory, so that it gets the same special treatment as
+is applied to the standard system directories.
+@ifset cppmanual
+@xref{System Headers}.
+@end ifset
+
 @item -fpreprocessed
 @opindex fpreprocessed
 Indicate to the preprocessor that the input file has already been

gnu/dist/gcc/gcc/doc/invoke.texi

@@ -294,6 +294,7 @@ in the following sections.
 -include @var{file}  -imacros @var{file} @gol
 -iprefix @var{file}  -iwithprefix @var{dir} @gol
 -iwithprefixbefore @var{dir}  -isystem @var{dir} @gol
+-isystem-cxx @var{dir} @gol
 -M  -MM  -MF  -MG  -MP  -MQ  -MT  -nostdinc  -P  -remap @gol
 -trigraphs  -undef  -U@var{macro}  -Wp,@var{option}}