Clarify how ObservedTimestamp field is set if unspecified (#3385)

This is a clarification that follows from how the data model is
specified:
https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/logs/data-model.md#field-observedtimestamp

This was noticed in a comment on another PR:

https://github.com/open-telemetry/opentelemetry-specification/pull/3383/files#r1164491423

Co-authored-by: jack-berg <34418638+jack-berg@users.noreply.github.com>

specification/logs/bridge-api.md

@@ -118,7 +118,8 @@ this API.
 The API MUST accept the following parameters:
 
 - [Timestamp](./data-model.md#field-timestamp)
-- [Observed Timestamp](./data-model.md#field-observedtimestamp)
+- [Observed Timestamp](./data-model.md#field-observedtimestamp). If unspecified the
+  implementation SHOULD set it equal to the current time.
 - The [Context](../context/README.md) associated with the `LogRecord`. The API
   MAY implicitly use the current Context as a default
   behavior.