docs: add GoDoc comments to query_insert.go exported API

[kyungseopk1m]

This PR adds GoDoc documentation to all exported methods in query_insert.go.

Part of #1283 (adding GoDoc comments to exported APIs).

Following the maintainer's suggestion to submit documentation incrementally, one file at a time.

Changes

• Added GoDoc comments to 20 exported InsertQuery methods:
  • Chainable builders: Conn, Model, Err, With, WithRecursive, WithQuery, Table, TableExpr, ModelTableExpr, Column, ColumnExpr, ExcludeColumn
  • WHERE conditions: Where, WhereOr
  • Operation metadata: Operation
  • Upsert helpers: On, Set, SetValues
  • Execution: Scan, Exec
• Style matches the polished comments added in docs: add GoDoc comments to db.go exported API #1313 (db.go) and docs: add GoDoc comments to query_select.go exported API #1316 (query_select.go).