filter-known-issues.py: clarify what "new" means
filter-known-issues.py: clarify what "new" means

When building incrementally, filter-known-issues.py reports a varying
and totally different set of "new" issues than when building from
scratch. Warnings for unrelated upstream code disappearing and
re-appearing are especially confusing. Expand the messages a bit to
clarify.

Signed-off-by: Marc Herbert <77e7c7d746b52c6eb01119b65a031ae1f650246d@intel.com>

GiulianoFranchetto/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,finikorg/zephyr,finikorg/zephyr,ldts/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,galak/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,ldts/zephyr,finikorg/zephyr,finikorg/zephyr,ldts/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,galak/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr