Document tuning BM25 k1 and b parameters

PR #4701 added per-field tunable BM25 k1 and b parameters via typmod, but this feature is not yet documented.

We should add documentation covering:

• How to set k1 and b per field via typmod syntax
• What each parameter controls (term frequency saturation, length normalization)
• Default values and recommended tuning ranges
• Examples of when to adjust these parameters

This is an internal issue to ParadeDB, not for community contributors