Since signals can take arguments, let's suffix them with () as we
already do with functions. To make sure we remain consistent, make the
`update-dbus-docs.py` script check & fix any occurrences where this is
not the case.
Resolves: #31002
This also adds an ignore list, which currently contains the whole API as of
version 250, since that's the base we used for dbus interfaces.
See d9d2d16aea