MINOR: sample: add the xxh3 converter

This patch adds support for the XXH3 variant of hash function that
generates a 64-bit hash.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 95b020c..ca4c581 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -16356,6 +16356,15 @@
   This prefix is followed by a name. The separator is a '.'. The name may only
   contain characters 'a-z', 'A-Z', '0-9', '.' and '_'.
 
+xxh3([<seed>])
+  Hashes a binary input sample into a signed 64-bit quantity using the XXH3
+  64-bit variant of the XXhash hash function. This hash supports a seed which
+  defaults to zero but a different value maybe passed as the <seed> argument.
+  This hash is known to be very good and very fast so it can be used to hash
+  URLs and/or URL parameters for use as stick-table keys to collect statistics
+  with a low collision rate, though care must be taken as the algorithm is not
+  considered as cryptographically secure.
+
 xxh32([<seed>])
   Hashes a binary input sample into an unsigned 32-bit quantity using the 32-bit
   variant of the XXHash hash function. This hash supports a seed which defaults