Clarify Env Variable usage for list-type URI params (#15345)

Clarify Env Variable usage for list-type URI params

docs/static/env-vars.asciidoc

@@ -10,6 +10,7 @@
 * You can give a default value by using the form `${var:default value}`. Logstash uses the default value if the
 environment variable is undefined.
 * You can add environment variable references in any plugin option type: string, number, boolean, array, or hash.
+* Environment variables for list-type URI parameters can support lists of space-delimited values. Currently, other non-URI based options do not support lists of values. See <<plugin-concepts>>
 * Environment variables are immutable. If you update the environment variable, you'll have to restart Logstash to pick up the updated value.
 
 ==== Examples