doc/README.PATCH-POLICY.SUSE: Update the summary and background
    (jsc#PED-5021)
    
    * Drop information about patches being split into directories per
      a subsystem because that is no longer the case.
    * Remove the mention that the expanded tree is present since SLE11-SP2
      as that is now only a historical detail.
    * Incorporate URLs and additional information in parenthenses directly
      into the text.
    * Fix typos and improve some wording.