What do you document in your Rails project?
I understand the general consensus is 'tests are your documentation', but a client I am working with wants some documentation in the code. I've decided to go with YARD, but I'm unsure of whether to document every method, or only certain methods or complicated lines.

Should I document in controllers where necessary? 

Should I insist on spending this time building more exhaustive tests?
What do you document in your Rails project?
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I understand the general consensus is 'tests are your documentation', but a client I am working with wants some documentation in the code. I've decided to go with YARD, but I'm unsure of whether to document every method, or only certain methods or complicated lines.

Should I document in controllers where necessary? 

Should I insist on spending this time building more exhaustive tests?