Wording of r.literal api page

Currently, the r.literal introduction is something like:

Replace an object in a field instead of merging it with an existing object in a merge or update operation. Using literal with no arguments in a merge or update operation will remove the corresponding field.

I think this is a bit confusing because it starts out by explaining what r.literal does, which is very complicated. I think a re-formulation indicating it acts more like a marker than like a function would be nice. Here's a stab at it:

A marker term that indicates to the update and merge terms that a field in an object should be replaced entirely by the value passed to r.literal, rather than being merged with it. If r.literal is not passed an argument, update and merge will remove the field from the resulting document.

Open the r.literal API page and compare its current introduction with the wording proposed in the issue. Update the introduction so it explains the marker behavior with update and merge, including the no-argument case, then verify the rendered page reads clearly.