man: document new "boot-complete.target" unit
    
    (cherry picked from commit 82ea38258c0f4964c2f3ad3691c6e4554c4f0bb0)
    
    Related: #1872243
    
    patch_name: 0433-man-document-new-boot-complete.target-unit.patch
    present_in_specfile: true
    location_in_specfile: 433
    squash_commits: true