Add documentation for tracer.Start() (open-telemetry#1864)

Signed-off-by: Anthony J Mirabella <a9@aneurysm9.com>

trace/trace.go

@@ -638,7 +638,19 @@ func (sk SpanKind) String() string {
 
 // Tracer is the creator of Spans.
 type Tracer interface {
-	// Start creates a span.
+	// Start creates a span and a context.Context containing the newly-created span.
+	//
+	// If the context.Context provided in `ctx` contains a Span then the newly-created
+	// Span will be a child of that span, otherwise it will be a root span. This behavior
+	// can be overridden by providing `WithNewRoot()` as a SpanOption, causing the
+	// newly-created Span to be a root span even if `ctx` contains a Span.
+	//
+	// When creating a Span it is recommended to provide all known span attributes using
+	// the `WithAttributes()` SpanOption as samplers will only have access to the
+	// attributes provided when a Span is created.
+	//
+	// Any Span that is created MUST also be ended. This is the responsibility of the user.
+	// Implementations of this API may leak memory or other resources if Spans are not ended.
 	Start(ctx context.Context, spanName string, opts ...SpanOption) (context.Context, Span)
 }