Add documentation on usage scenarios for span processors

sdk/lib/opentelemetry/sdk/trace/export/batch_span_processor.rb

@@ -13,6 +13,9 @@ module Export
         # Implementation of the duck type SpanProcessor that batches spans
         # exported by the SDK then pushes them to the exporter pipeline.
         #
+        # Typically, the BatchSpanProcessor will be more suitable for
+        # production environments than the SimpleSpanProcessor.
+        #
         # All spans reported by the SDK implementation are first added to a
         # synchronized queue (with a {max_queue_size} maximum size, after the
         # size is reached spans are dropped) and exported every

sdk/lib/opentelemetry/sdk/trace/export/simple_span_processor.rb

@@ -12,6 +12,12 @@ module Export
         # {Span} to {io.opentelemetry.proto.trace.v1.Span} and passes it to the
         # configured exporter.
         #
+        # Typically, the SimpleSpanProcessor will be most suitable for use in testing;
+        # it should be used with caution in production. It may be appropriate for
+        # production use in scenarios where creating multiple threads is not desirable
+        # as well as scenarios where different custom attributes should be added to
+        # individual spans based on code scopes.
+        #
         # Only spans that are recorded are converted, {OpenTelemetry::Trace::Span#is_recording?} must
         # return true.
         class SimpleSpanProcessor