doc: update sanitycheck doc to clarify some details
doc: update sanitycheck doc to clarify some details

Add details about that fact that ram/flash params on the board yaml are
specified in terms of Kilobytes.  Also what the defaults are if they are
not specified.

Clarified that ignore_tags is meant for ignoring something from both
build and running. (I can see adding a tag for tests we build, but
ignore that we can run).

Signed-off-by: Kumar Gala <a5e5248af4cd4f0ed8c515f61d40a6e2db46a66e@linaro.org>

finikorg/zephyr,aceofall/zephyr-iotos,punitvara/zephyr,finikorg/zephyr,explora26/zephyr,aceofall/zephyr-iotos,fractalclone/zephyr-riscv,zephyriot/zephyr,mbolivar/zephyr,runchip/zephyr-cc3220,Vudentz/zephyr,holtmann/zephyr,aceofall/zephyr-iotos,GiulianoFranchetto/zephyr,explora26/zephyr,rsalveti/zephyr,GiulianoFranchetto/zephyr,aceofall/zephyr-iotos,mbolivar/zephyr,galak/zephyr,nashif/zephyr,mbolivar/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,galak/zephyr,Vudentz/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,runchip/zephyr-cc3220,rsalveti/zephyr,Vudentz/zephyr,holtmann/zephyr,mbolivar/zephyr,fractalclone/zephyr-riscv,galak/zephyr,nashif/zephyr,punitvara/zephyr,ldts/zephyr,runchip/zephyr-cc3220,ldts/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,aceofall/zephyr-iotos,holtmann/zephyr,fbsder/zephyr,holtmann/zephyr,nashif/zephyr,fractalclone/zephyr-riscv,kraj/zephyr,runchip/zephyr-cc3220,kraj/zephyr,runchip/zephyr-cc3220,ldts/zephyr,nashif/zephyr,punitvara/zephyr,rsalveti/zephyr,fractalclone/zephyr-riscv,zephyriot/zephyr,fbsder/zephyr,finikorg/zephyr,fbsder/zephyr,holtmann/zephyr,zephyriot/zephyr,Vudentz/zephyr,zephyriot/zephyr,rsalveti/zephyr,explora26/zephyr,ldts/zephyr,mbolivar/zephyr,explora26/zephyr,nashif/zephyr,fractalclone/zephyr-riscv,zephyrproject-rtos/zephyr,fbsder/zephyr,galak/zephyr,zephyriot/zephyr,kraj/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,fbsder/zephyr,punitvara/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,rsalveti/zephyr,explora26/zephyr,kraj/zephyr