I'm embarrassed to have only realized the correct form very recently (after some resistance in a code review). More specifically, I've always made entities plural by using an apostrophe (e.g., "This field holds a list of SomeCoolObject's."). I think my train of thought was that it differentiates (better?) when the entity name happens to end with an 's'.
There are standard typographical conventions I don't always adhere to in the context of talking about code or other technical subjects. For example, the American standard is for punctuation that comes at the end of a quotation should be inside the closing quotation mark, "like this." I sometimes do the reverse, or even put a space before a period or comma, just to avoid possible confusion about what's actually part of the quotation and what's not.
The apostrophe in the way you're using it here is something similar: you're accentuating that the actual name of the thing you're talking about is SomeCoolObject. Sometimes that kind of thing could matter (particularly think of when pluralizing a word changes its spelling beyond simply adding an "s" to the end). You might be better off just restructuring a sentence to avoid having to pluralize at all: "a list with members of type SomeCoolObject" or somesuch.
This is known as the greengrocer's apostrophe. Most sources will say it's wrong, but it's quite common if you look for it. I think it's fine, particularly when talking about code when the actual name of the symbol matters.
