docs: document metadata filter operators on v2 search methods

[Can Şahin (Cnashn)]

Summary

The filter parameter on the v2 AsyncPGVectorStore search methods accepts a rich metadata filter DSL ($eq, $ne, $gt, $gte, $lt, $lte, $in, $nin, $between, $exists, $like, $ilike, $and, $or, $not, plus nested a.b paths), but the method docstrings were one-line summaries that never mentioned filter or the supported operators. Users end up guessing the syntax and running into surprises (e.g. #276, #304).

This documents it:

• asimilarity_search gets a full Args/Returns docstring listing every supported operator with a short example.
• asimilarity_search_with_score gets a concise Args block that references asimilarity_search for the filter syntax.

Notes

• Docs only, no behavior change.
• Every filter example is taken straight from the existing FILTERING_TEST_CASES, so the documented syntax matches tested behavior.
• I also noted that comparison and membership operators expect scalar metadata values rather than JSON arrays, which is the specific limitation behind the confusion in Similarity search with distance filter failed: (psycopg.errors.UndefinedColumn) column "product_ids" does not exist #304.

Checks

• ruff format --diff and ruff check pass
• mypy passes
• tests/unit_tests/v2/test_async_pg_vectorstore_search.py: 159 passed