libbson has plenty of helper macros to allow skipping passing -1 arguments and have the underlaying function run strlen() on the argument.

These macros aren't really well documented.
We do have a blurb about them at http://mongoc.org/libbson/current/creating.html

Libbson provides macros to....

but it would be better if we would document the BSON_APPEND_UTF8 macro on the same page as we document bson_append_utf8