Instead of a confirmation or dialog, how about a simple text message in, black or red, preferrably small font size, that appears above or below the Doc Type drop-down whenever it is changed without Doc Text first receiving any changes, that says "Doc Text must also be changed to save", or some similar message?

Alternatively, Bugzilla could present a message after a Bugzilla is saved, at the top, saying that the Doc Type changes were not saved because Doc Text was left unchanged. I'm not sure whether Bugzilla places warning or error messages in that location or not.

Although I recognize that the current modus operandi is a bit confusing, one of the alternatives--allowing Doc Text to be saved independently of a Doc Type change--would probably lead to incorrectly-submitted text, even if a dialog presented the message in comment 1 and asked whether to proceed.

(In reply to comment #2)
> Instead of a confirmation or dialog, how about a simple text message in,
> black or red, preferrably small font size, that appears above or below the
> Doc Type drop-down whenever it is changed without Doc Text first receiving
> any changes, that says "Doc Text must also be changed to save", or some
> similar message?

This is probably the best approach. Won't work with non JS enabled browsers, but that is the price you pay.