rustdoc: improve legibility/spacing between multi-paragraph items

[virtualritz]

See the screenshot below.

As you can see, the first line of the next list item, Interactive Rendering and Scene Edits, Fault Tolerance, etc. is too close to the last paragraph of the previous item.

I.e. spacing between paragraphs under a list item is bigger than spacing between list items. It should be at least the same.

Here is the section of source that generates this:

//! Implements a renderer for the node-based image compositing language API,
//! `nci`.
//!
//! # Design Principles
//!
//! * Simplicity
//!
//!   The interface itself should be simple to understand and use, even if
//!   complex things can be done with it. This simplicity is carried into
//!   everything which derives from the interface.
//!
//! * Interactive Rendering and Scene Edits
//!
//!   Scene edit operations should not be a special case. There should be no
//!   difference between scene description and scene edits. In other words, a
//!   scene description is a series of edits and vice versa.
[...]