design-autorepair.rst: clarify tag precedence and conflict

This commit clarifies one particular point of the auto-repair workflow:
what to do when multiple, conflicting administrator-set tags exist in an
object; and how tags at different levels (cluster, node group and instance)
interact.

For conflict within an object, we choose to always let the most restrictive
tag win (i.e. the least destructive repair, and the longest suspension
time). For tags at different levels, we follow a simple "nearest tag wins"
rule.

Signed-off-by: Dato Simó <dato@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>