Enhancement: Improve WP_CACHE Constant Language in Jetpack Boost Cache Settings

### Impacted plugin

Boost

### What

When the WordPress `WP_CACHE` constant needs to be updated, we say it is an "easy" fix. It likely isn't easy for many folks as it involves editing the `wp-config.php` file

### How


We link to this doc:

- [WP_CACHE defined but not true.](https://jetpack.com/support/jetpack-boost/troubleshooting-caching-issues/#wp-cache-defined-not-true)

Start by reviewing the Jetpack Boost Cache Settings language shown in the issue and the linked “WP_CACHE defined but not true” documentation. Update the description so it does not call editing wp-config.php an “easy” fix and accurately sets expectations; done means the wording is clearer and remains consistent with the linked guide.

Written by the indexing model from the issue text.