The writer here is confusing things. An apostille is a guarantee of the validity of a signature. In California it can be done by county clerks. (see: http://www.sos.ca.gov/notary/authentication/).
A proof of pension is usually from Social Security office. However, other documents can be accepted. For example, my SS was too small so I got the CEO of a small limited partnership I have some small shares in to write up a fancy letter telling how much guaranteed income I got from them. We got the signature notarized, affixed a fancy stamp and it was accepted like that. As a bonus I also wrote up a certification, in Spanish, that all of the cited documents were true and accurate, had it notarized and stamped and included it. It seemed like a silly thing to do but we have found that having lots of official-looking documents with fancy stamps and certifications makes things easier.