doc: release 3.6: no CACHE_MANAGEMENT by default on stm32h7/f7

Mentioning that stm32h7 and stm32F7 series do not have the
CONFIG_CACHE_MANAGEMENT by default. The application must
explicitly set CONFIG_CACHE_MANAGEMENT=y to activate
the cache (Icache, Dcache) on those stm32 series.

Signed-off-by: Francois Ramu <francois.ramu@st.com>
(cherry picked from commit 1c59fa52)