mention NORET, with caveats

git-svn-id: https://svn.r-project.org/R/trunk@87759 00db46b3-68df-0310-9c12-caf00c1e9a41

Author: ripley

doc/manual/R-exts.texi

@@ -15179,6 +15179,14 @@ These are defined in header @file{R_ext/Error.h} included by @file{R.h}.
 @strong{NB}: when @code{R_NO_REMAP} is defined (as is done for
 C++ code), @code{Rf_error} etc must be used.
 
+Header @file{R_ext/Error.h} defines a macro @code{NORET} intended to be
+used only from C code (C++ code can use the @code{[[noreturn]]}
+attribute).  This covers various ways to signal to the compiler that the
+function never returns.  Because the usages of those ways differ by C
+standard, it should always be used at the beginning of a function
+declaration, including before @code{static} and attributes such as
+@code{attribute_hidden}.
+
 
 @c 1-item menu: kept to avoid spurious warning from makeinfo 6.8/7.0
 @menu