Document monotonicity of dmu_tx_assign() and txg_hold_open()

Expand the comments to make it clear exactly what is guaranteed
by dmu_tx_assign() and txg_hold_open(). Additionally, update
the comment which refers to txg_exit() when it should reference
txg_rele_to_sync().

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Christian Schwarz <me@cschwarz.com>
Closes #11521