Adds documentation and javadocs about SpanAdjuster and SpanReporter

fixes #570

docs/src/main/asciidoc/spring-cloud-sleuth.adoc

@@ -329,6 +329,9 @@ SpanAdjuster customSpanAdjuster() {
 
 This will lead in changing the name of the reported span just before it gets sent to Zipkin.
 
+IMPORTANT: Your `SpanReporter` should inject the `SpanAdjuster` and
+ allow span manipulation before the actual reporting is done.
+
 === Host locator
 
 In order to define the host that is corresponding to a particular span we need to resolve the host name

spring-cloud-sleuth-core/src/main/java/org/springframework/cloud/sleuth/SpanAdjuster.java

@@ -19,6 +19,9 @@ package org.springframework.cloud.sleuth;
 /**
  * Adds ability to adjust a span before reporting it.
  *
+ * IMPORTANT: Your {@link SpanReporter} should inject the {@link SpanAdjuster} and
+ * allow {@link Span} manipulation before the actual reporting is done.
+ *
  * @author Marcin Grzejszczak
  * @since 1.1.4
  */

spring-cloud-sleuth-core/src/main/java/org/springframework/cloud/sleuth/SpanReporter.java

@@ -19,6 +19,9 @@ package org.springframework.cloud.sleuth;
 /**
  * Contract for reporting Sleuth spans for collection. For example to Zipkin.
  *
+ * IMPORTANT: Your {@link SpanReporter} should inject the {@link SpanAdjuster} and
+ * allow {@link Span} manipulation before the actual reporting is done.
+ *
  * @author Marcin Grzejszczak
  * @since 1.0.0
  */