zpool-features(7): Note that the boot loader has support for large_blocks

Since r304321 (-current: Aug 18, 2016) and r328866 (stable/11: Feb 5, 2018)
the FreeBSD loader has supported reading from datasets with the
large_blocks feature active.

PR: 247992
Reported by: Anton Saietskii <vsasjason@gmail.com>
MFC after: 2 weeks
Sponsored by: Klara Inc.