sd-bus/man: document EBUSY error in bus_message_read (#21954)

sd-bus/man: document EBUSY error in bus_message_read (#21954)

* sd-bus/man: document EBUSY error in bus_message_read

The EBUSY error can be returned from sd_bus_exit_container(), and, if
that happens, it will be propogated upwards towards bus_message_read. In
terms of documentation, this means that bus_message_read's man page
can't just include the error text for sd_bus_message_read_basic, as
reading basic types exclusively doesn't have the potential for this
error.

sd_bus_message_read_basic's error documentation isn't incorrect when
applied to sd_bus_message_read, it's just incomplete.  While EBUSY is
documented in sd_bus_message_open_container.xml,
it's explanation is unique to the sd_bus_message_exit_container function
and makes for poor documentation of the general read API.