man: split out sd_bus_set_fd() man page from sd_bus_get_fd()

sd_bus_get_fd() and related calls are useful for integrating a bus
connection into arbitrary event loops. But sd_bus_set_fd() is quite a
different beast, it's for using D-Bus over pre-initialized sockets or
pairs of fifos or stuff, i.e. very advanced stuff.

Let's split this man page in two, in order not to confuse things
needlessly.

And while we are at it, let's slightly extend the documentation.