ci: update spelling

[tonyandrewmeyer]

The docs spellcheck seems to have updated underneath us, so make adjustments so that the checks pass again.

For the most part, I've added words to the custom word list, but I've made a few adjustments to the docs in cases where it seemed better than adding the word.

The PR also corrects the description of Scenario's behaviour when creating a temporary charm root: only the metadata files are written - the src and lib folders are not also copied there.

(See also canonical/jubilant#225)

[benhoyt]

Ah, thanks for this. Yeah, I noticed this over in Jubilant-land too. It's another thing that makes me wonder about the worthwhileness of spelling checks. But we can get David's opinion on all that when he's back.

[dwilding]

Thanks for doing this @tonyandrewmeyer. The extra spelling exceptions were needed because the spelling check previously wasn't checking all text. I was tracking the fix in canonical/documentation-style-guide#138 (sorry, I think I forgot to mention that in my notes before I went away).

I don't expect major changes to the spelling setup from now on, now that (as far as I could test previously) it's checking everything that it should be checking.