Update documentation to remove 'desirable' suggestion for what is not a clear cut approach

[AvnerCohen]

I suggest to remove the desirable comment.

I have been supporting large, small and huge I18n'ed systems. I came to clearly conclude that using plain text format/YAMLs as the backend for the translated data is the way to go, it makes things far superior in:

1. Auditing, change control and versions
2. Scaling translation with external services is easier with YML and flat file, rather than dumping and re-inserting data to tables.
3. Environment management - no need to move data from dev to prod in the form of insert scripts, it's all part of the code base.

I could well be wrong, but at the very least, if not whole-heartily recommending using simple YAMLs as I18n storage, I think it would be more helpful to drop the "desirable" references to (questionable) alternatives.

[rails-bot]

r? @arthurnn

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