Improve auto-clear cache Dashboard message

[raamdev]

When Quick Cache runs the auto_clear_cache() routine, which clears the entire site cache, the following Dashboard message is displayed:

Since only major changes should be triggering something is extreme as clearing the entire site cache, I feel that we should update this message to say "Quick Cache: detected major site changes. Found cache files for this site (auto-clearing)."

[raamdev]

Decided that "important" is a better word than "major", as the latter may come across as "many things changing on the site", when that may not be the case. However, "important" site changes simply indicate that they're important enough to Quick Cache to warrant a full clearing of the cache.

[raamdev]

Next release changelog:

• Enhancement: Improved Dashboard messaging for the auto_clear_cache() routine. See #328.