Illumos #3447 improve the comment in txg.c

3447 improve the comment in txg.c

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Richard Lowe <richlowe@richlowe.net>
Reviewed by: Garrett D'Amore <garrett@damore.org>
Reviewed by: Richard Elling <richard.elling@dey-sys.com>
Approved by: Dan McDonald <danmcd@nexenta.com>

References:
  illumos/illumos-gate@adbbcfface63b3a71922d5a25d34a2018c0435de
  https://www.illumos.org/issues/3447

Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>