New bindings:
`gr` - `helpful-update`, reload helpful buffer.
`SPC m q` - `helpful-kill-buffers`, kill all helpful buffers.
Move `o` binding definition into separate post-init function, so the binding
is defined only if the `link-hint` package is actually installed.
Add documentation of the added bindings.
The helpful layer delivers more sophisticated help buffers,
at least for elisp and emacs specific objects. Therefore
it makes sense to replace the existing help functions for
these bindings silently if this layer is loaded.
