Word is doing something I haven't seen before.

In Print Layout view, there are now dotted lines marking
the vertical and horizontal margins of my document. There
are L-shaped brackets at all four corners of content on
the page.

These showed up in one document today -- I dunno why --
and now all my Word docs are doing the same thing.

What are they? How do I make them go away? And come back?

It's a neat little feature, but an annoyance as long as I
can't explain it.