Shotwell stores the following information in its own database: tag, title, rating, event and photo transformation (have I missed anything?). It also has the option to "Write tags, titles, and other metadata to photo files".
If I select this option, is all of the metadata stored in the file? If not, which metadata is not stored in the file?
Based on this page on their wiki, it appears that Shotwell can only write keywords, titles, ratings, and orientation. I've used all these and I don't know of any other user modifiable fields (besides adjusting date & time) that Shotwell can do. It looks like all the metadata that Shotwell can modify is written to the original file, if you enable that option in the preferences.
Here is another page from their wiki in which they are looking at metadata and interoperability with other programs and services. If any other fields become user-modifiable, they will probably be documented there first.
There is at least one Shotwell developer here using Ask Ubuntu, and you will probably get a more definitive answer soon. It looks like the original wording you quoted in your question just included a catch-all term that was not specific enough.
tl/dr YES, Any metadata you edit with Shotwell is written to the file, as of the current version.
