Doc: Add workaround for field ref parser rejecting valid input data

[karenzone]

Update Troubleshooting section in Logstash Reference to include work around for issue #11608.

Related content:

• https://www.elastic.co/guide/en/logstash/current/configuration-file-structure.html#field-reference
• https://www.elastic.co/guide/en/logstash/current/field-references-deepdive.html

Related issue: #11608

[karenzone]

cc:/@robbavey @yaauie