Dealing with Word spell check in technical documents?

Judging from your image I'd say the best way to deal with that would be to use styles properly. You can set individual styles to be ignored when spell-checking which is very handy for things like source code or random nonsense. This can be found on the "Format" button when editing styles.

I'd think that for actual words that appear in your text you would probably want to add them to the dictionary instead of ignoring them over and over again each session.

But I'm assuming here, that your uses of unknown words are pretty confined, in this case to code blocks, examples, listings, etc. This may or may not apply to your case. You certainly won't find a dictionary containing all possible directory names (referring to your screenshot again, here).

Try some of the 'Options' mentioned in the lower left hand corner - there's a lot of choices of things to Ignore.

You can also consider 'Add to Dictionary' instead of 'Ignore' - it'll add the words to the dictionary, which will let you (eventually) create your own dictionary of common technical words.