The documentation for Boottime concludes with "(since Linux 4.17; identical to 'Monotonic')", which sounds as if this behaves the same as Monotonic on a modern linux system. That is not the case, both experimentally and based on reading the clock_gettime(2) man page on my linux system here, which states:

   CLOCK_BOOTTIME (since Linux 2.6.39; Linux-specific)
          A nonsettable system-wide clock that is identical  to
          CLOCK_MONOTONIC,  except  that  it  also includes any
          time that the system is suspended.  This  allows  ap‐
          plications  to  get  a  suspend-aware monotonic clock
          without having to  deal  with  the  complications  of
          CLOCK_REALTIME, which may have discontinuities if the
          time is changed using settimeofday(2) or similar.

I suspect that the parenthetical got into the documentation accidentially and it should just be removed.