Update OpenCensus bridge docs with limitations

bridge/opencensus/doc.go

@@ -12,5 +12,24 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-// Package opencensus provides a migration bridge forwarding the OpenCensus API to the OpenTelemetry SDK.
+// Package opencensus provides a migration bridge from OpenCensus to
+// OpenTelemetry. The NewTracer function should be used to create an
+// OpenCensus Tracer from an OpenTelemetry Tracer. This Tracer can be use in
+// place of any existing OpenCensus Tracer and will generate OpenTelemetry
+// spans for traces. These spans will be exported by the OpenTelemetry
+// TracerProvider the original OpenTelemetry Tracer came from.
+//
+// There are known limitations to this bridge:
+//
+// - The AddLink method for OpenCensus Spans is not compatible with the
+// OpenTelemetry Span. No link can be added to an OpenTelemetry Span once it
+// is started. Any calls to this method for the OpenCensus Span will result
+// in an error being sent to the OpenTelemetry default ErrorHandler.
+//
+// - The NewContext method of the OpenCensus Tracer cannot embed an OpenCensus
+// Span in a context unless that Span was created by that Tracer.
+//
+// - Conversion of custom OpenCensus Samplers to OpenTelemetry is not
+// implemented. An error will be sent to the OpenTelemetry default
+// ErrorHandler if this is attempted.
 package opencensus // import "go.opentelemetry.io/otel/bridge/opencensus"