docs: RocksDB flush-ERROR spam at shutdown when collection directory is removed while handle is open

[s2x]

Finding (from #192 follow-up)

Removing a collection directory (e.g. rm -rf in test cleanup) while the collection handle is still open causes ~10 console ERROR lines from RocksDB at process shutdown (rocksdb_context.cc:349, inverted_indexer.cc:102, id_map.cc:76) as the destructor tries to flush against the missing directory.

Evidence

• Observed empirically during fetch(): expose includeVector + mirror upstream nullable normalization #192 test development: a .phpt test that deleted its temp dir in finally before calling close() produced the ERROR spam in output, breaking --EXPECT-- matching.
• Calling close() before directory removal avoids it entirely.

Current state

Documented in docs/helpers/faq.md (added with #192): tests must close() (or destroy()) before removing the directory. An explicit section in README/docs ("Lifecycle: close before cleanup") may help library users too — this issue tracks that documentation work.

Low priority — footgun is documented; this is about user-facing docs beyond the helper knowledge base.