docs(trace): describe self.upstream as lazily built on first access

[MilagrosMarin]

Post-#1499 (shipped in v2.3.1), self.upstream is no longer constructed eagerly before every make() — the framework records the key, and Diagram.trace(self & key) is built the first time the user accesses self.upstream and memoized for the rest of the call. make() implementations that never read self.upstream pay nothing.

The shipped code (src/datajoint/autopopulate.py:135-141, :673-674) and the release-notes performance highlight both describe this behavior, but two docs paragraphs still describe the pre-#1499 lifecycle.

Changes

src/reference/specs/trace.md — §"Where it's set"

• "constructs self.upstream = Diagram.trace(self & key) immediately before invoking" → "records the current key but defers constructing the trace; built on first access, memoized for the rest of the call".
• Preserves the SQL-lazy / fetch-not-cached wording (still accurate).

src/reference/specs/autopopulate.md — §4.4 self.upstream behavior

• Lifecycle bullet: same substitution as above.
• Lazy bullet: adds the "at most once" and "make() calls that never read upstream pay nothing" nuance.

[dimitri-yatsenko]

LGTM. Accurately describes the lazy self.upstream behavior merged in #1499 — key recorded before make(), trace built on first access and memoized, never built if unused, cleared in finally.