Clean up D-Bus documentation (bug#41744)

Clean up D-Bus documentation (bug#41744)

* doc/lispref/errors.texi (Standard Errors): The error symbol
dbus-error is defined even when Emacs is built without D-Bus.

* doc/misc/dbus.texi (Bus Names, Introspection)
(Nodes and Interfaces, Methods and Signal)
(Properties and Annotations, Arguments and Signatures)
(Synchronous Methods, Receiving Method Calls, Signals)
(Alternative Buses, Errors and Events): Clarify wording.  Fix
indentation of and simplify examples where possible.  Improve
Texinfo markup and cross-referencing where possible.
(Type Conversion): Ditto.  Remove mentions of Emacs' fixnum range
now that we have bignums.

* lisp/net/dbus.el (dbus-return-values-table)
(dbus-call-method-asynchronously, dbus-send-signal)
(dbus-register-signal, dbus-register-method)
(dbus-string-to-byte-array, dbus-byte-array-to-string)
(dbus-escape-as-identifier, dbus-check-event, dbus-event-bus-name)
(dbus-event-message-type, dbus-event-serial-number)
(dbus-event-service-name, dbus-event-path-name)
(dbus-event-interface-name, dbus-event-member-name)
(dbus-list-activatable-names, dbus-list-queued-owners, dbus-ping)
(dbus-introspect-get-interface-names, dbus-introspect-get-interface)
(dbus-introspect-get-method, dbus-introspect-get-signal)
(dbus-introspect-get-property, dbus-introspect-get-annotation-names)
(dbus-introspect-get-annotation, dbus-introspect-get-argument-names)
(dbus-introspect-get-argument, dbus-introspect-get-signature)
(dbus-set-property, dbus-register-property)
(dbus-get-all-managed-objects, dbus-init-bus): Clarify docstring and
improve formatting where possible.
(dbus-call-method): Ditto.  Remove mentions of Emacs' fixnum range
now that we have bignums.