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

[donnchawp]

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/

[github-actions]

This issue has been marked as stale. This happened because:

• It has been inactive for the past 6 months.
• It hasn’t been labeled `[Pri] BLOCKER`, `[Pri] High`, `[Type] Feature Request`, `[Type] Enhancement`, `[Type] Janitorial`, `Good For Community`, `[Type] Good First Bug`, etc.

No further action is needed. But it's worth checking if this ticket has clear reproduction steps and it is still reproducible. Feel free to close this issue if you think it's not valid anymore — if you do, please add a brief explanation.