Request: Update user_manual with documentation about testmode doesn't translate variables

[bronius]

Please forgive my ignorance.. I tried without success to create a simple PR to include my recommended addition to documentation to help wayward developers after me to not burn an hour and a half the way I just did: https://github.com/bronius/documentation/commit/c2c63238c68d3108054b96e6f2f58f7f2357a62f#diff-229131ed0af7cf80ed3a9470ec7beb4c Just a note that when operating in testmode, there's no way to know whether the recipient-variables and template recipient variable placeholders are translating as expected.

Thanks! -Bronius