Comment and restrict change of large blocks support in replication

It was recently found (https://github.com/openzfs/zfs/pull/10383) that support of large blocks in replication can not be changed after initial replication was done.  On 11.3 it causes data loss on files with large blocks whenever the large blocks replication support is changed from enabled to disabled or back.  In 12 the data loss was fixed, but attempt to replicate with disabled large blocks support after its being enabled will fail.

We should document these issues, plus in 12 block disabling of the large blocks support after initial replication (or task creation), plus in 11.3 deny any changes to it after initial replication (or task creation).  We'll see whether the fix can be merged into 11.3, but it is not trivial, so lets try to workaround it so far.