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 <kumar.gala@linaro.org>