sdk/log: Document that users should not create instances of Record

[pellared]

Users are expected to emit logs using this API: https://pkg.go.dev/go.opentelemetry.io/otel/log (or a bridge using it).

The SDK is not designed to create https://pkg.go.dev/go.opentelemetry.io/otel/sdk/log#Record themselves. The record is created by the Logger returned by https://pkg.go.dev/go.opentelemetry.io/otel/sdk/log#LoggerProvider.Logger.

Originally posted by @pellared in #5782 (comment)

[pellared]

We should document that users should not create instances of https://pkg.go.dev/go.opentelemetry.io/otel/sdk/log#Record by themselves and that https://pkg.go.dev/go.opentelemetry.io/otel/sdk/log/logtest#RecordFactory can be used for testing purposes.