Can you point out an example where the documentation is missing or insufficient?
Core packages are usually well documented. And third party packages, well, depend on the author of course.
There is no assumption that any functionality is “just like ST” or other editors. That is to say: it’s not assumed that you know ST, neither that any given functionality is the same, even if it happens to be.
The ability to use snippets is part of core Atom. But which ones you decide to write or use, depends on you or any third party package implementing them. So if you’re looking for documentation on specific snippets, check first where they come from.
However, how snippet syntax works is well documented in the docs (linked by @leedohm).
Hope that helps.
I think the overall answer to your question is this:
Atom supplies capabilities, much more than specific implementations making use of them.
In this case Atom gives you snippet capability and instructions how to write them. What you or a package author make of it, is up to you/them.