Clarify semantics of return from deserialize

[tslocke]

This can trip folks up who are using compile-to-JS languages (ClojureScript in my case), where return semantics may be different.

Is this adding or improving a feature or fixing a bug?

What's the new behavior?

How does this change work?

Have you checked that...?

• The new code matches the existing patterns and styles.
• The tests pass with yarn test.
• The linter passes with yarn lint. (Fix errors with yarn prettier.)
• The relevant examples still work. (Run examples with yarn watch.)

Does this fix any issues or need any specific reviewers?

Fixes: #
Reviewers: @

[ianstormtaylor]

Thanks @tslocke!

(I agree this is not the least confusing API. Maybe we’ll find a better solution in the future.)