doc/README.SUSE: Update information about module support status
    (jsc#PED-5759)
    
    Following the code change in SLE15-SP6 to have externally supported
    modules no longer taint the kernel, update the respective documentation
    in README.SUSE:
    
    * Describe that support status can be obtained at runtime for each
      module from /sys/module/$MODULE/supported and for the entire system
      from /sys/kernel/supported. This provides a way how to now check that
      the kernel has any externally supported modules loaded.
    
    * Remove a mention that externally supported modules taint the kernel,
      but keep the information about bit 16 (X) and add a note that it is
      still tracked per module and can be read from
      /sys/module/$MODULE/taint. This per-module information also appears in
      Oopses.