[SIEM] Update Maps error message/docs to detail that matching index patterns are required

[spong]

When the map can't locate a Kibana Index Pattern that matches one of the index globs configured in the siem:defaultIndex advanced setting the Required index patterns not configured error is displayed.

With the current message displayed it is not obvious that the user must have a Kibana Index Pattern that exactly matches one of the siem:defaultIndex indices for the map to load. Adding this detail to the error message and/or Map configuration help link would aid in situations where users specify their own indices outside the typical beats setup/configuration, and are not seeing the map load.

[elasticmachine]

Pinging @elastic/siem (Team:SIEM)

[FrankHassanabad]

"Reviewed by Frank Hassanabad on 7/29/2020", This looks complete and will be shippable at this point. Going to close this and associated tickets. Any new bugs/features should be new tickets.