khan: document %peek overlay
** external peek interface in arvo, at arm 22. (lyc is always
** [~ ~], i.e. request from self.)
**
+** the %peek runtime overlay functions like an extra arm of the
+** $%. the full accepted type would be something like:
+**
+** $+ each path
+** $% [%once vis=view syd=desk tyl=spur]
+** [%beam vis=view bem=beam]
+** [%urth *]
+** ==
+**
** %move is a kernel move. these are injected into arvo, except
** again for a runtime overlay.
**
