ingest-user-agent: code/documentation mismatch for regex_file option

[AE777F63]

Elasticsearch version (bin/elasticsearch --version):
Version: 7.2.0, Build: oss/tar/508c38a/2019-06-20T15:54:18.811730Z, JVM: 11.0.1

Plugins installed: []

JVM version (java -version):
openjdk 11.0.1 2018-10-16
OpenJDK Runtime Environment 18.9 (build 11.0.1+13)
OpenJDK 64-Bit Server VM 18.9 (build 11.0.1+13, mixed mode)

Description of the problem including expected versus actual behavior:
The documentation states that regex_file is supposed to have .yaml extension, but apparently it has to be .yml. Setting regex_file to "regexes.yaml" does not work; "regexes.yml" does.

https://github.com/elastic/elasticsearch/blob/7.4/docs/reference/ingest/processors/user-agent.asciidoc#using-a-custom-regex-file
https://github.com/elastic/elasticsearch/blob/7.2/modules/ingest-user-agent/src/main/java/org/elasticsearch/ingest/useragent/IngestUserAgentPlugin.java#L72

Steps to reproduce:

1. Put regexes.yaml to /usr/share/elasticsearch/config/ingest-user-agent/regexes.yaml
2. Set regex_field to "regexes.yaml" in pipeline configuration
3. Start elasticsearch and see that it does not load regexes.yaml

[elasticmachine]

Pinging @elastic/es-docs (>docs)

[elasticmachine]

Pinging @elastic/es-core-features (:Core/Features/Ingest)