Add release note for double msvc init fix.

Author: grafikrobot

doc/src/history.adoc

@@ -9,6 +9,12 @@ user code to handle the conbination of turning off exceptions while treating
 warnings as errors, and getting warnings/errors from the std library by having
 the users silence the warning themselves.
 
+This patch also fixes the case of asking to initialize any msvc toolset
+versions (`using msvc ;`) when there are already versions initialized. Instead
+of erroring to say that a version is already in use, it considers the set
+of already initialized msvc toolsets as satisfying the request to generally
+initialize msvc.
+
 == Version 5.2.0
 
 Many fixes in this release from regular contributors Nikita and Dmitry. There