Add warning that pages need single root element

Editor assessment

The subject states the warning and its rule exactly — pages need a single root element — so the header communicates the docs addition and the misconception it prevents without the diff, and the docs type fits perfectly. Hygiene is clean: imperative mood, no trailing period, sensible length, no ticket junk. No body is needed for a focused warning whose subject carries the rule, and no footer is required. A line on the rendering failure that motivates the rule would add depth; the message is already clear, accurate, and publishable.