Boost: Shorten Cache notice descriptions


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.

Start by searching the Boost codebase for the cache notice descriptions and the existing "learn more" link. Compare the displayed notice copy with the shorter example in this issue; done means the description is concise, the link is separated as requested, and any affected notice wording is consistent.

Written by the indexing model from the issue text.