help.lint.checks: check for disposable leaks and print more details when...

A patch with two improvements to help-lint.

The first one, kind of suggested by you :), is to check if the `$example` leaks disposables. E.g `100 <buffer>` gives the error "1 disposable(s) leaked in example". 

Second changes `check-values` error from "$values don't match stack effect" to e.g "$values don't match stack effect; expected { "x" "y" }, got { "a" "b" }" which makes it clearer why it complains.