Update release notes for 5.3.0.

Author: grafikrobot

doc/src/history.adoc

@@ -3,6 +3,10 @@
 
 == Version 5.3.0
 
+Biggest changes for this release is the creation of two new toolsets: `ibmcxx`
+and `cc`. The former covers all the IBM compilers, and the latter is a generic
+C compiler toolset.
+
 * *New*: Replace the old `vacpp` and `xlcpp` with a new `ibmcxx` toolset that
   supports the old compilers and the new "IBM XL" and  "IBM Open XL" compilers.
   -- _René Ferdinand Rivera Morell_ and _Chris Ward_
@@ -36,6 +40,9 @@
   built-in `-h` help.
   -- _René Ferdinand Rivera Morell_
 
+WARNING: The `vacpp` and `xlcpp` have been removed in favor of `ibmcxx` as the
+latter covers the same support.
+
 == Version 5.2.1
 
 This patch reverts the change to define `_HAS_EXCEPTIONS=0` for Dinkumware