update_wordlist() claims that it will remove from WORDLIST words from SPELLING_WORDLIST

[Bisaloo]

This is a follow up of #80 that I finally got around to testing.

When running update_wordlist(), it will ask to remove words from SPELLING_WORDLIST that are not used in the project. Turns out it doesn't really remove them since it only touches WORDLIST and not the custom, additional, wordlist.

I believe the current behaviour to not touch SPELLING_WORDLIST is desirable but the message from update_wordlist() may need to be updated to reflect what's really happening.