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.
    
    (cherry picked from commit a1a03fa54bfb45315eefaa49ceb38a21aceafde8)