docs: apply Tina Engel's corrections to agent document proposal

Incorporates all corrections from PR mendixlabs#214 (tinaengel-mx):
- Fix BSON field name: CustomDocumentType → DocumentType
- Clarify Excluded field: user-controllable toggle, not pre-release artifact
- Model type field: "" → "Text generation"
- Model metadata: decoded from key, not Portal-pushed on Test Key click
- KnowledgeBase key: update sample UUID/qualifiedName (LLMKey → KnowledbaseKey)
- ConsumedMCPService: add endpoint (String constant ref) and
  authenticationMicroflow (optional, no params, returns List<HttpHeader>)
- Agent: add microflow tool example (toolType: "Microflow", document ref)
- Agent: add minSimilarity to KB tool, temperature/topP/toolChoiceToolName fields
- UsageType: Conversational → Chat throughout
- Block keyword: MCP SERVICE → CONSUMED MCP SERVICE throughout
- Go structs: add ToolChoiceToolName, MinSimilarity, Endpoint,
  AuthenticationMicroflow; remove speculative Microflow field from ToolRef
- Catalog: add endpoint_constant, authentication_microflow columns
- Open Question mendixlabs#2 marked answered (microflow tool shape confirmed)
- Risk table: microflow tool shape row resolved

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: ako