Deprecate diagonal tooltip classes

As discussed in #14938, the classes `.top-left`, `.top-right`, `.bottom-left`, and `.bottom-right` were added as the first step for automatic diagonal placement of tooltips, but the corresponding JavaScript was never added.

X-Ref: #7961 and https://github.com/twbs/bootstrap/commit/88ece4fc5902f7dd9582fb4e5956062fe97af84c

Assuming that we don't want to add the JS for these specific classes more than a year later, they should therefore be deprecated.

/cc @cvrebert @hnrch02 
/fyi @mdo @fat