The documentation says:

> The Builder type is used to create a BSON document. The type only allows the iterative building of a document, so there is no way to verify the contents outside of writing the document. If you have a Builder and need to conditionally add a field, you can write the document to a byte slice and use the Reader type to lookup the desired document, but in this case you should probably use a Document instead.

Hoever there is no `Builder` type defined anywhere in that package, which is misleading.