[doc]Added advice for production env variables

[ObjectiveTruth]

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Had an issue that seemed silly, so I updated the docs with it explicitly. Basically you can't name your Env variables the same as the variables

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[gmethvin]

s/withing/within/

[gmethvin]

Is it worth mentioning this other special case? https://github.com/typesafehub/config#note-about-resolving-substitutions-in-referenceconf-and-applicationconf

[ObjectiveTruth]

Is it worth mentioning this other special case? https://github.com/typesafehub/config#note-about-resolving-substitutions-in-referenceconf-and-applicationconf

I don't think so, because that's more a best practice than a gotcha. If the user is expecting support for programmatic operations in their *.conf file, that probably means they don't understand what the *.conf files are used for, or will look up more info themselves.