Add guidance for excluding subfolders for folder sources

[joshcanhelp]

I found this guidance in the Obsidian forums and it worked for what I needed. I thought it would be helpful for others (and wanted to confirm this was the best way to accomplish it).

[holroy]

Instead of saying To exclude subfolders, use the file.folder property. which kind of indicates that using the file.folder is excluding, could we word it more like:

To exclusively look in this particular folder, and not subfolders, we can use the `file.folder` property like shown below.

And possibly also re-add the FROM "projects/brainstorming" since that is done at an earlier level of the query, and could affect the overall performance of the query. This could have a huge impact on a larger vault.