Add information about the snapshot repository (#27719)

From https://twitter.com/ddanieltwitt/status/938754761968451585, I think
we should add in our documentation, where users can find the SNAPSHOT
versions we are publishing.

docs/java-rest/high-level/getting-started.asciidoc

@@ -46,6 +46,9 @@ Central]. The minimum Java version required is `1.8`.
 The High Level REST Client is subject to the same release cycle as
 Elasticsearch. Replace the version with the desired client version.
 
+If you are looking for a SNAPSHOT version, the Elastic Maven Snapshot repository is available
+at https://snapshots.elastic.co/maven/.
+
 [[java-rest-high-getting-started-maven-maven]]
 ==== Maven configuration
 

docs/java-rest/low-level/sniffer.asciidoc

@@ -21,6 +21,8 @@ released with `5.0.0-alpha4`. There is no relation between the sniffer version
 and the Elasticsearch version that the client can communicate with. Sniffer
 supports fetching the nodes list from Elasticsearch 2.x and onwards.
 
+If you are looking for a SNAPSHOT version, the Elastic Maven Snapshot repository is available
+at https://snapshots.elastic.co/maven/.
 
 ==== Maven configuration
 
@@ -131,4 +133,4 @@ than from Elasticsearch:
 --------------------------------------------------
 include-tagged::{doc-tests}/SnifferDocumentation.java[custom-hosts-sniffer]
 --------------------------------------------------
-<1> Fetch the hosts from the external source
+<1> Fetch the hosts from the external source

docs/java-rest/low-level/usage.asciidoc

@@ -22,6 +22,9 @@ released with `5.0.0-alpha4`. There is no relation between the client version
 and the Elasticsearch version that the client can communicate with. The
 low-level REST client is compatible with all Elasticsearch versions.
 
+If you are looking for a SNAPSHOT version, the Elastic Maven Snapshot repository is available
+at https://snapshots.elastic.co/maven/.
+
 [[java-rest-low-usage-maven-maven]]
 ==== Maven configuration