Fix rerank docstring/comment accuracy (PR review)

Two doc-accuracy fixes from the PR #205 review:

- The cohere.py module docstring said ScoredDocument.document is null on
  every result, but the parser passes a document echo through when a
  response carries one (fixture 012 + unit tests). Reword to match.
- The response.py comment said undeclared extras are forwarded to the
  wire untouched, but the mapping reserves model / query / documents /
  top_n. Tighten to note the provider-reserved keys.

Author: chris-colinsky

src/openarmature/retrieval/providers/cohere.py

@@ -16,11 +16,12 @@
 provider appends ``/v2/rerank``), overridable for a proxy / private gateway.
 The ``transport`` parameter is the test seam (``httpx.MockTransport``).
 
-The Cohere ``/v2/rerank`` wire has no ``return_documents`` parameter and
-never echoes document text, so
+The Cohere ``/v2/rerank`` wire has no ``return_documents`` parameter, so
 ``RerankRuntimeConfig.return_documents`` is a silent no-op on this wire (the
-mapping sends no wire field for it and leaves ``ScoredDocument.document`` null
-on every result). ``max_tokens_per_doc`` rides the extras pass-through bag.
+mapping sends no wire field for it). Cohere v2 does not echo document text, so
+``ScoredDocument.document`` is null in practice; the parser still passes
+through a ``document`` echo when a response carries one (a compatible gateway
+or a fixture). ``max_tokens_per_doc`` rides the extras pass-through bag.
 """
 
 from __future__ import annotations

src/openarmature/retrieval/response.py

@@ -160,8 +160,10 @@ class RerankResponse(BaseModel):
 
 # Spec §2 rerank runtime config: one declared field ``return_documents``
 # (boolean, default False) plus the extras pass-through bag (``extra="allow"``).
-# Undeclared fields supplied by callers are forwarded to the wire body
-# untouched by the §8 wire-format mapping.
+# Undeclared fields supplied by callers are forwarded to the wire body by the
+# §8 wire-format mapping, except for the provider-reserved keys a mapping
+# manages itself (e.g. the Cohere mapping owns model / query / documents /
+# top_n, so a caller extra cannot clobber them).
 class RerankRuntimeConfig(BaseModel):
     """Per-call rerank request parameters."""