Supporting documents to show at port of entry in US for a job interview on B1/B2 visa

A B1/B2 visa (specifically the "B1" part) is absolutely the correct visa type for entering the US for a job interview. This is true regardless of whether the job itself is located in the US or elsewhere.

As with all entries to the US under a non-immigrant visa, the burden of proof is on you to show that you will depart the US before your status expires, and that you do not intent to "work" (for the immigration definition of work) whilst you are in the US.

If you state that you are in the US for a job interview (and you should state that - if you are asked) then you can expect the immigration staff to set a higher burden of proof on you than if you were there for a holiday. Your obviously aware of this fact, and it's why you're specifically stating that what additional documents you will need.

The most important document you can provide will be a letter from your prospective employer stating that they are inviting you for an interview, but making it very clear that they will NOT be employing you on this trip, and that if they do decide they offer you a job they will be responsible for obtaining all required documentation (visas, etc). If your potential employer has hired people under similar circumstances in the past then they will likely have a standard letter they will provide for this purpose.

Secondary proof would include a return ticket in the near future (ie, showing that you are only planning to stay in the US for a few days, and not several months), a hotel booking matching the timeframe of your visit, plus potentially anything you have to show current ties to your home country, such as up-to-date rent/ownership statements for a house, proof of your current job (if possible showing that you are on a short-term leave from there covering the period of the trip to the US), or anything else that could assist in showing that you do intend to leave the US.

Most importantly, you should make it clear to the border patrol staff that you are visiting for an 'interview', and NOT for a 'job' or to 'work', and that you do NOT intent to stay in the US on this trip.

I have traveled to the US myself for a job interview, and whilst in that case the job itself was outside of the US the moment I mentioned "interview" the CBP staff did ask for further proof, such as what I've described above (specifically flight and hotel details, and a letter from the potential employer). Once they were confident that I would leave the country after my interview I was allowed enter.

Going for a job interview is a perfectly normal thing, and is allowed on a B1 visa. However as @HonoraryWorldCitizen says, saying that you are going for a job interview demonstrates intent to immigrate, and thus can lead to you being denied entry. (Don't get me started on the wild illogicality of the current state of US immigration practice).

A number of legal sites advise that you are vague about your intentions, saying things like "here to meet with potential business colleagues", without lying if you are asked directly about a job interview. As for the other things, bring all the usual stuff that indicates your willingness to return after the interview and go through normal immigration channels: return ticket, documents indicating your current employment, etc.