Document vue-boostrap upgrade gotchas

When the vue-boostrap update will land in gitlab-ui we may have few hiccups in the tests due to how BTooltip and BPopup works

• The reason to set attachToDocument to true is that the BTooltip and BPopover directives expect to be attached to a document object.
• The reason to set sync to false is that otherwise, specs for components that use BTooltip or BPopover will fail with the following error:

TypeError: Cannot read property 'sync' of null

While we did / are fixing all the test that we have currently in place new tests may be written and engineers may find it confusing, we should so add few lines to the doc to document this gotcha,