Boost: Shorten Cache notice descriptions

[dilirity]

The description of our notices is way too big. I think we can replace it with a short sentence and put the "learn more" link on a new paragraph. We could move the description to the support page or remove it altogether.

For example, the description above can be shortened to this:

This feature cannot be enabled because your site is already using a caching system, installed by another plugin or your hosting provider.