Index target no date math for data stream nor alias

👋 Regarding elastic/docs-content#2761 & #1252, 

Attempts to reduce doc pain point surface area to explicitly note to users to stop using date math with aliases or data streams. 

Aliases will not ILM rollover bootstrap & data streams will bootstrap but incur multiple data streams which then fail to progress through ILM rollover as expected.

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Author: stefnestor

docs/index.asciidoc

@@ -842,10 +842,15 @@ NOTE: `ilm_rollover_alias` does NOT support dynamic variable substitution as
 The indexing target to write events to. 
 Can point to an {ref}/index-mgmt.html[index], {ref}/aliases.html[alias], or {ref}/data-streams.html[data stream]. 
 This can be dynamic using the `%{foo}` syntax.
+
+Dynamic syntax might make sense with data streams and aliases, but results should be
+validated against cluster index templates and Index lifecycle management (ILM) setup.
+Placeholder date math should not normally be used with aliases nor data streams.
+
 The default value will partition your indices by day so you can more easily
 delete old data or only search specific date ranges.
 Indexes may not contain uppercase characters.
-For weekly indexes ISO 8601 format is recommended, eg. logstash-%{+xxxx.ww}.
+For weekly indexes ISO 8601 format is recommended, eg. `logstash-%{+xxxx.ww}`.
 Logstash uses
 http://www.joda.org/joda-time/apidocs/org/joda/time/format/DateTimeFormat.html[Joda
 formats] and the `@timestamp` field of each event is being used as source for the date.