doc: Standard library header units

It seems users are confused by the lack of standard library header
units.

	gcc/
	* doc/invoke.texi (C++ Modules): Document lack of std
	library header units.

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 054b83715934e0b021d83b7e07eded4b91548ccd..8766bcdfc1876168d60bf934220586255f8ef372 100644 (file)
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -32891,6 +32891,13 @@ the purview of a named module.  This is not implemented.  Such
 declarations will be attached to the module, if any, in which they are
 declared.
 
+@item Standard Library Header Units
+The Standard Library is not provided as importable header units.  If
+you want to import such units, you must explicitly build them first.
+If you do not do this with care, you may have multiple declarations,
+which the module machinery must merge---compiler resource usage can be
+affected by how you partition header files into header units.
+
 @end table
 
 Modular compilation is @emph{not} enabled with just the