Review polish: drop unused prefix machinery, tighten comments and docs

- Remove the registry's unused prefix=True slot, its span_for() branch,
  its doc-rendering case and its test - nothing in the stack sets it.
- Stop promising a "later phase" query-duration metric dimension in the
  db.operation.name description; the cardinality rationale stands alone.
- Replace baked-in benchmark numbers in the telemetry module docstring
  with the docs' own phrasing (below run-to-run variation).
- Compact the duplicated copy_context() and enqueue-site comments in
  database.py to pointers at their canonical tellings.
- Make the "catch people out" gotchas skimmable as a bullet list and
  give the changelog's "nothing is removed" line a clear antecedent.
- Add a test that a result cut short by max_returned_rows records
  datasette.truncated=True - previously only ever asserted False.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012U7coQfVu8nK2R4q2mCULA

docs/telemetry_doc.py

@@ -26,8 +26,7 @@ def spans(cog):
 
     cog.out("\n")
     for span in SPANS:
-        title = f"{span}*" if span.prefix else str(span)
-        cog.out(f"``{title}``\n")
+        cog.out(f"``{span}``\n")
         cog.out(f"    {span.description}\n\n")
         # INTERNAL is the default and the overwhelming majority of spans -
         # printing it on every one would be noise. Only the exceptional case,