Remove Boost.System from the list of libraries requiring separate compilation

more/getting_started/detail/header-only.rst

@@ -33,7 +33,6 @@ The only Boost libraries that *must* be built separately are:
 * Boost.Regex_
 * Boost.Serialization_
 * Boost.Signals_
-* Boost.System_
 * Boost.Thread_
 * Boost.Timer_
 * Boost.Wave_
@@ -65,4 +64,8 @@ A few libraries have optional separately-compiled binaries:
   which requires a separately-compiled binary. This is enabled by
   #define BOOST_ENABLE_NON_INTRUSIVE_EXCEPTION_PTR.
 
+* Boost.System_ is header-only since Boost 1.69. A stub library is
+  still built for compatibility, but linking to it is no longer
+  necessary.
+
 __ ../../libs/graph/doc/read_graphviz.html