docs: document single argument variant of tracingBaggageToTag (#3247)

Implementation allows single argument variant which
is used in the wild so document it.

Follow up on #1160 and #1168

Signed-off-by: Alexander Yastrebov <alexander.yastrebov@zalando.de>

docs/reference/filters.md

@@ -3303,11 +3303,19 @@ Syntax:
 tracingBaggageToTag("<baggage_item_name>", "<tag_name>")
 ```
 
-Example: If a trace consists of baggage item named `foo` with a value `bar`. Adding below filter will add a tag named `baz` with value `bar`
+Example:
+
+If a trace consists of a baggage item named `foo` with a value of `bar`,
+then adding the filter below will add a tag named `baz` with the value `bar`:
 ```
 tracingBaggageToTag("foo", "baz")
 ```
 
+A single-argument variant uses the baggage item name as the tag name, i.e., it will add a tag named `foo` with the value `bar`:
+```
+tracingBaggageToTag("foo")
+```
+
 ### stateBagToTag
 
 This filter sets an opentracing tag from the filter context (state bag).