Remove some comments related to pqPipelineSync() and PQsendPipelineSync()

These comments explained how these functions behave internally, and the
equivalent is described in the documentation section dedicated to the
pipeline mode of libpq.  Let's remove these comments, getting rid of the
duplication with the docs.

Reported-by: Álvaro Herrera
Reviewed-by: Álvaro Herrera
Discussion: https://postgr.es/m/202401150949.wq7ynlmqxphy@alvherre.pgsql

src/interfaces/libpq/fe-exec.c

@@ -3247,23 +3247,6 @@ PQsendPipelineSync(PGconn *conn)
 /*
  * Workhorse function for PQpipelineSync and PQsendPipelineSync.
  *
- * It's legal to start submitting more commands in the pipeline immediately,
- * without waiting for the results of the current pipeline. There's no need to
- * end pipeline mode and start it again.
- *
- * If a command in a pipeline fails, every subsequent command up to and
- * including the result to the Sync message sent by pqPipelineSyncInternal
- * gets set to PGRES_PIPELINE_ABORTED state. If the whole pipeline is
- * processed without error, a PGresult with PGRES_PIPELINE_SYNC is produced.
- *
- * Queries can already have been sent before pqPipelineSyncInternal is called,
- * but pqPipelineSyncInternal needs to be called before retrieving command
- * results.
- *
- * The connection will remain in pipeline mode and unavailable for new
- * synchronous command execution functions until all results from the pipeline
- * are processed by the client.
- *
  * immediate_flush controls if the flush happens immediately after sending the
  * Sync message or not.
  */