Added short description of sub-query functionality (#12038)

* Added short description of sub-query functionality

Smuggled in a bunch of minor copy changes.

* Emphasize sub-query support limitation

Co-Authored-By: flamber <1447303+flamber@users.noreply.github.com>

* Escape text that looks like Jekyll variables

Co-Authored-By: flamber <1447303+flamber@users.noreply.github.com>

* s/it/them/

Co-Authored-By: Maz Ameli <maz@metabase.com>

* Add note about sub-queries having to be in the same db

Co-Authored-By: Maz Ameli <maz@metabase.com>

Co-authored-by: flamber <1447303+flamber@users.noreply.github.com>
Co-authored-by: Maz Ameli <maz@metabase.com>