Improve documentation of mach_*write_*compressed()
mtr0log.ic, mtr0log.h:
  Improve documentation of mlog_write_initial_log_record_fast()


innobase/include/mtr0log.h:
  Improve documentation of mlog_write_initial_log_record_fast()
innobase/include/mtr0log.ic:
  Improve documentation of mlog_write_initial_log_record_fast()
innobase/include/mach0data.h:
  Improve documentation of mach_*write_*compressed()
innobase/include/mach0data.ic:
  Improve documentation of mach_*write_*compressed()