[docs] Add wait_until_yellow to fix build failure

The snippet in the docs creates and index and uses it with the
_analyze api. The trouble is that if the index hasn't been created
fully the _analyze API will fail. This adds a
GET _cluster/health?wait_for_status=yellow
which fixes the issue.

While this does make the docs more cluttered, it also makes the snippets
actually runnable.

Closes #18165

docs/reference/analysis/analyzers/pattern-analyzer.asciidoc

@@ -103,6 +103,8 @@ PUT test?pretty=1
   }
 }
 
+GET _cluster/health?wait_for_status=yellow
+
 GET test/_analyze?analyzer=camel&text=MooseX::FTPClass2_beta
 # "moose","x","ftp","class","2","beta"
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