docs: clarify BM25 is keyword retrieval, not semantic search in unit 2.1

[UdayKiranJadi]

What this fixes

Closes #319

The PartyPlanningRetrieverTool example uses BM25Retriever, which is a
lexical/keyword-based retrieval algorithm — not semantic search. The code
and surrounding prose incorrectly labeled it as "semantic search", which
confused learners (as noted in the issue thread).

Changes

• Replaced "vector database... semantic search" description with accurate
  BM25/keyword-based explanation, including a note on how it differs from
  vector-based semantic search
• Fixed "combines predefined knowledge with semantic search" →
  "keyword-based retrieval and LLM reasoning"
• Fixed tool description string: "Uses semantic search" →
  "Uses keyword-based retrieval (BM25)"

[UdayKiranJadi]

Hi! I've submitted a PR to fix this — #697 . Please take a look when you get a chance!"

[yuka-with-data]

I noticed the PR went with Option 1 (from issue #319), and nice work on the update.

I was also curious how the maintainers would prefer this to evolve, since the surrounding section already starts leaning into vector databases. Option 2 could also fit that flow naturally, but I am fine with either direction depending on the intended framing.

Either way, this is a solid improvement. Thanks for working on it.