Boost & WPSC: "advanced-cache.php" may be hidden by host

When a user is removing Boost or WPSC they may need to delete the file wp-content/advanced-cache.php. I just found out that some hosts hide that file, and they have to "show hidden files" to view it. We should update documentation to reflect that possible situation.
ref: https://wordpress.org/support/topic/cannot-locate-file-for-manual-deletion/

Start by locating the Boost and WPSC removal documentation that tells users to delete wp-content/advanced-cache.php, then check the linked support report for context. Update the instructions to mention that some hosts hide advanced-cache.php and that users may need to enable “show hidden files”; done means the warning appears in the relevant removal guidance.

Written by the indexing model from the issue text.