man: be clearer that .timer time expressions need to be reset to override them
    
    let's be clearer about the overriding concept for OnCalendar= settings.
    
    Prompted by this thread:
    
    https://lists.freedesktop.org/archives/systemd-devel/2019-March/042351.html
    (cherry picked from commit 58031d99c6320855b86f4890baa9165597e3d841)
    
    Resolves: #1816908
    
    patch_name: 0372-man-be-clearer-that-.timer-time-expressions-need-to-.patch
    present_in_specfile: true
    location_in_specfile: 372
    squash_commits: true