Make sure it is very clear we are not building a Logging API (#2966)

We are only building a logging "backend" which is supposed to be used via appenders by existing logging libraries. Additionally, we may provide helpers to build "events" that make easier to build LogRecords with specific semantic convention. Such helpers will also call the logging "backend" APIs.

We do not intend to have user-facing logging API in the spec.

Resolves #2936

specification/logs/api.md

@@ -25,6 +25,12 @@
 
 </details>
 
+<b>Note: this document defines a log *backend* API. The API is not intended to be called
+by application developers directly. It is provided for logging library authors
+to build [Appenders](#how-to-create-log4j-style-appender), which use
+this API to bridge between existing logging libraries and the OpenTelemetry log
+data model.</b>
+
 The Logs API consist of these main classes:
 
 * LoggerProvider is the entry point of the API. It provides access to Loggers.