DSL doc: uninstall now available for all Casks

But required for `pkg`-based Casks.   This has been true for a little
while, but not documented.  The previous situation (silent failure unless
an `install` stanza was present) was also not documented.

This PR
* Also contains some general cleanup in section "Uninstall Stanza Details",
 which still needs work.
* Will merge conflict with #6121