Clarify ineligible translation scenario

[mjankowski]

Resolves #1330

Notes:

• There is a linked issue there around discovering user language, which maybe should be dealt with as well. This narrowly fixes the "we don't explain why this error can happen" portion, which is the main point of the issue.
• This also doesn't challenge the issue of whether the API should act this way (ie, should we intercept/override same-language translation requests?) and merely documents what actually occurs.