doc: twister: Clarify that the minimums are rough

There is no way to know for sure the size of an image built for a test,
given that zephyr can build for so many different architectures,
platforms, software supports, etc. So it should be clear that the filter
for min_ram and min_flash on the twister testcases is an estimate, and
not a strictly well known value.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>