replace a comment about zfs recv -F corner case with a longer, more detailed one

The old comment in zfs_rezget explains what situation the code handles,
the new comment also describes how the situation can arise.

Also, re-join a line that became sufficiently shorti some time ago.

Differential Revision: https://reviews.freebsd.org/D2352
Reviewed by: delphij, smh
MFC after: 12 days