Correct docstring

`map_field_names_and_values()` is a function, not a method, and its docstring
is being updated accordingly.
Add check_typed_fields() function to _update()

This is a function that can be used to enforce that fields are of the correct
type. It can be used within `_update()` and will raise `TypeError` for any
field that isn't of the right type.

This needs tests!

References #18
Add AttributeError to docstring for _update()

Through `map_field_names_and_values()`, `_update()` can raise `AttributeError`.
Its docstring only mentioned `TypeError`, which `_update()` raises outside of
any internal functions.