docs: Document how functions translate to the client versions in dag and how they differ

[levlaz]

@vikram-dagger, I'm also thinking we need to document somewhere (not in this PR, of course) how these functions translate to the client versions in dag, and how they differ. Several Go users have been confused by this, especially when a function returns both an error and an object, because the client version doesn't return an error in this case.

Originally posted by @helderco in #7379 (comment)