Update release notes for 1.2.1

Author: msimberg

docs/sphinx/releases/whats_new_1_2_1.rst

@@ -15,15 +15,15 @@ General changes
 
 This is a bugfix release. It contains the following changes:
 
-* Fix compilation on ARM.
-* Fix compilation on s390x.
-* Fix compilation on 32-bit architectures.
+* Fix compilation on ARM, s390x and 32-bit architectures.
 * Fix compilation with Boost 1.69.0.
+* Fix a critical bug in the ``future`` implementation.
 
 Closed issues
 =============
 
 * :hpx-issue:`3638` - Build HPX 1.2 with boost 1.69
+* :hpx-issue:`3635` - Non-deterministic crashing on Stampede2
 * :hpx-issue:`3550` - 1>e:\000work\hpx\src\throw_exception.cpp(54): error C2440: '<function-style-cast>': cannot convert from 'boost::system::error_code' to 'hpx::exception'
 * :hpx-issue:`3549` - HPX 1.2.0 does not build on i686, but release candidate did
 * :hpx-issue:`3511` - Build on s390x fails
@@ -32,6 +32,7 @@ Closed issues
 Closed pull requests
 ====================
 
+* :hpx-pr:`3666` - Fixing yet another race in future_data
 * :hpx-pr:`3663` - Fixing race between setting and getting the value inside future_data
 * :hpx-pr:`3648` - Adding timestamp option for S390x platform
 * :hpx-pr:`3647` - Blind attempt to fix warnings issued by gcc V9