Add changes, reference; fix trailing whitespace

doc/system/changes.adoc

@@ -9,16 +9,51 @@ http://www.boost.org/LICENSE_1_0.txt
 
 [#changes]
 # Release History
-:idprefix: changes_
+:idprefix:
 
 ## Changes in Boost 1.69
 
-...
+* Boost.System is now header-only. A stub library is still built for
+  compatibility, but linking to it is no longer necessary.
+* Even more functions have been marked `constexpr`.
+* The destructor of `error_category` is now protected and no longer
+  virtual. This is a _potentially breaking change_ but its impact
+  is expected to be limited.
+* `error_category` now has a constructor that accepts a 64 bit identifier,
+  enabling distinct category objects to compare equal.
+* The constructors of `error_category` are now protected.
+* A non-allocating, nonthrowing overload of `message` has been added.
+* A virtual function `failed` has been added, allowing categories for
+  which success is not synonymous with 0.
+* The deprecated `boost::system::throws` object has been removed.
+* `boost::throws()` is now deprecated and its use is discouraged.
 
 ## Changes in Boost 1.68
 
-...
+On a {cpp}14 compiler, many Boost.System functions and member functions
+are now `constexpr`, and `error_code` and `error_condition` are literal
+classes.
+
+In addition to enabling use in constant expressions (and `constexpr`
+functions), this significantly improves the quality of the generated code.
+
+As a result of this change, however, now using Boost.System from {cpp}14
+or {cpp}17 code requires that the library is also built with {cpp}14 or
+above. This is the default on GCC 6 and newer, but not on GCC 5 or Clang.
+One can build Boost for {cpp}14 by passing the `cxxstd=14` option to `b2`.
+
+(Previous versions allowed code built against any {cpp} standard to link
+with Boost.System built against any {cpp} standard. In 1.68, code using
+any {cpp} standard can link with Boost.System built with {cpp}14 or above,
+but if Boost.System is built with {cpp}11 or below, only code also built
+with {cpp}11 and below can link to it successfully.)
 
 ## Changes in Boost 1.65
 
-...
+On a {cpp}11 compiler, Boost.System now provides implicit conversions
+from `boost::system::error_category`, `error_code`, and `error_condition`
+to their standard equivalents from `<system_error>`.
+
+This allows libraries to expose a {cpp}11 interface and report errors
+via `std::error_code` even when using Boost.System, directly or through a
+dependency such as Boost.ASIO.