Knowledge Advanced has two authoring spell check features.

1 - The authoring spell check function which appears as a button at the bottom of the article being edited.

2 - The browser spell check function which is enabled in the authoring configuration - Browser-based Spellcheck.

Resolution:

Both Spell Checks can be used.

The authoring spell check allows you to use spell check in the specific language of the article. Some knowledge authors may need to edit articles in different locales or translate articles as part of their work responsibility. However this spellcheck can not be altered or improved by the user with new words.  

The browser spell check, depending on which function used by the browser may have more words defined.  It can also be used as the user is editing the article real time.  Depending on the user's browser you may need to control-right-click on the word to see the spell corrections.

When the browser spellcheck is enabled, the user has access to both options.