doc: clarify that a sublist needs a blank line

[dereuromark]

Problem

The behavior is fully specified, but split across two distant sections, and the connection is easy to miss.

The Block syntax preamble states the governing rule:

Paragraphs can never be interrupted by other block-level elements, and must always end with a blank line.

The List item section, however, only describes lazy continuation and says an item is "one or more lines, indented relative to the list marker." A reader there naturally expects

- a
  - b

to nest. It does not: because the indented - b would interrupt the open paragraph a, it folds in as lazy continuation, so this is a single-item list. A blank line is required to nest:

- a

  - b

This trips up newcomers (and is a common point of confusion when comparing djot to Markdown/CommonMark, where the first form nests).

Change

Add a short example to the List item section that makes the consequence of the non-interruption rule explicit, contrasting the no-blank-line case (single item) with the blank-line case (nested sublist), and pointing back to the Block syntax rule.

Verified against the reference (djot.js 0.3.2):

• - a / - b produces one <li> containing the paragraph.
• - a / blank / - b produces a nested <ul>.

Docs only: edits doc/syntax.md; doc/syntax.html is regenerated from it via the make rule (not committed here). Opened as a draft for your review of wording and placement.