Sync docs from 2.1.x to gh-pages

2.1.x/multi/multi__introduction.html

@@ -111,6 +111,7 @@ To do so, you have to do the following (for readability, we pass the dependencie
 						"span": "%X{X-B3-SpanId:-}",
 						"parent": "%X{X-B3-ParentSpanId:-}",
 						"exportable": "%X{X-Span-Export:-}",
+						"baggage": "%X{key:-}",
 						"pid": "${PID:-}",
 						"thread": "%thread",
 						"class": "%logger{40}",

2.1.x/single/spring-cloud-sleuth.html

@@ -111,6 +111,7 @@ To do so, you have to do the following (for readability, we pass the dependencie
 						"span": "%X{X-B3-SpanId:-}",
 						"parent": "%X{X-B3-ParentSpanId:-}",
 						"exportable": "%X{X-Span-Export:-}",
+						"baggage": "%X{key:-}",
 						"pid": "${PID:-}",
 						"thread": "%thread",
 						"class": "%logger{40}",

2.1.x/spring-cloud-sleuth.xml

@@ -4,7 +4,7 @@
 <book xmlns="http://docbook.org/ns/docbook" xmlns:xl="http://www.w3.org/1999/xlink" version="5.0" xml:lang="en">
 <info>
 <title>Spring Cloud Sleuth</title>
-<date>2020-02-19</date>
+<date>2020-02-20</date>
 <author>
 <personname>
 <firstname>Adrian Cole, Spencer Gibb, Marcin Grzejszczak, Dave Syer, Jay Bryant</firstname>
@@ -313,6 +313,7 @@ To do so, you have to do the following (for readability, we pass the dependencie
 						"span": "%X{X-B3-SpanId:-}",
 						"parent": "%X{X-B3-ParentSpanId:-}",
 						"exportable": "%X{X-Span-Export:-}",
+						"baggage": "%X{key:-}",
 						"pid": "${PID:-}",
 						"thread": "%thread",
 						"class": "%logger{40}",