Clarify requesting all stats in node stats docs

This commit clarifies how to explicitly obtain all stats from the node
stats API.

Author: jasontedor

docs/reference/cluster/nodes-stats.asciidoc

@@ -74,7 +74,7 @@ curl -XGET 'http://localhost:9200/_nodes/stats/process'
 curl -XGET 'http://localhost:9200/_nodes/10.0.0.1/stats/process'
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-The `all` flag can be set to return all the stats.
+All stats can be explicitly requested via `/_nodes/stats/_all` or `/_nodes/stats?metric=_all`.
 
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