Update revision history

build.jam

@@ -21,4 +21,5 @@ explicit
     ;
 
 call-if : boost-library system
+    : install boost_system
     ;

doc/system/changes.adoc

@@ -11,6 +11,10 @@ https://www.boost.org/LICENSE_1_0.txt
 ## Changes in Boost 1.91
 
 * The return type of `operator|(result<T&>, U)` has been changed to non-reference.
+* Pointers to members in `r & f` are now supported (by using `boost::compat::invoke`).
+* A CMake config file is now installed, even though the library is header-only. This
+  avoids breaking third-party `CMakeLists.txt` files that contain
+  `find_package(Boost COMPONENTS system ...)`.
 
 ## Changes in Boost 1.89