Document configuration for non-European languages for Index Backends

We got some questions about problems with index backends when a non-European language is used, for example in our Gitter chat and in these issues:

- #940
- #922
- #1416

The answer is usually to configure [a custom analyzer](https://docs.janusgraph.org/0.3.1/field-mapping.html#_custom_analyzer) that supports their language. We could add the use case of a property with values for a non-European language to our docs to make this more clear.

Start with the custom analyzer section in the linked field-mapping documentation and review issues #940, #922, and #1416 for the reported non-European-language use cases. Add a representative property example showing the required language-supporting analyzer, and confirm the documentation clearly explains the configuration.

Written by the indexing model from the issue text.