In the troff output, this doesn't seem to make any difference. But in the
html output, the whitespace is sometimes preserved, creating an additional
gap before the following content. Drop it everywhere to avoid this.
Let's systematically make sure that we link up the D-Bus interfaces from
the daemon man pages once in prose and once in short form at the bottom
("See Also"), for all daemons.
Also, add reverse links at the bottom of the D-Bus API docs.
Fixes: #34996
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
Zbigniew Jędrzejewski-Szmek
Lennart Poettering
Frantisek Sumsal
David Tardon
Abderrahim Kitouni
Oleg Solovyov
Yu Watanabe