Update release notes about armv8.6 build flag change

Partially Resolves: COMPMID-5346
Signed-off-by: Ramy Elgammal <ramy.elgammal@arm.com>
Change-Id: Ida1ad7ff19837203b3366466ba8018386cd6b18f
Reviewed-on: https://review.mlplatform.org/c/ml/ComputeLibrary/+/8065
Benchmark: Arm Jenkins <bsgcomp@arm.com>
Tested-by: Arm Jenkins <bsgcomp@arm.com>
Reviewed-by: SiCong Li <sicong.li@arm.com>
Comments-Addressed: Arm Jenkins <bsgcomp@arm.com>
diff --git a/docs/user_guide/release_version_and_change_log.dox b/docs/user_guide/release_version_and_change_log.dox
index 3c37b4f..1eeb111 100644
--- a/docs/user_guide/release_version_and_change_log.dox
+++ b/docs/user_guide/release_version_and_change_log.dox
@@ -62,6 +62,7 @@
  - Add fixed-format BF16, FP16 and FP32 Neon™ GEMM kernels to support variable weights.
  - Add new winograd convolution kernels implementation and update the ACL \link arm_compute::cpu::CpuWinogradConv2d CpuWinogradConv2d\endlink operator.
  - Add experimental support for native builds for Windows on Arm®.
+ - Build flag interpretation change: arch=armv8.6-a now translates to -march=armv8.6-a CXX flag instead of march=armv8.2-a + explicit selection of feature extensions.
  - Build flag change: toolchain_prefix, compiler_prefix:
    - Use empty string "" to suppress any prefixes.
    - Use "auto" to use default (auto) prefixes chosen by the build script. This is the default behavior when unspecified.