A, unless you package and distribute the code of wiki.js in some way with the application - but it sounds like you are talking about a hosted only solution so that won't be an issue.
If you allow self-hosting of your proprietary code by your users and the refers to your documentation server than this would still not trigger AGPL for the other code - you are not distributing wiki.js or parts of it. If you allow self hosting of the other code and include wiki.js for a local copy of your documentation then APGL might trigger.
Linking is meaning software linking, not referencing your own content displayed by the software. Think of it as the same difference between including using chunks of a paper in your own paper, or saying "as discussed in This Paper About This Thing (Him, Her, et. al, 2013)...". Or, as in this case the content is your's, referencing your own paper.
This potential confusion with the word "linking" (as natural languages are fairly dynamic beasts) is why people are sometimes overly fearful of AGPL and GPL (and often even LGPL) software.
If you allow self-hosting of your proprietary code by your users and the refers to your documentation server than this would still not trigger AGPL for the other code - you are not distributing wiki.js or parts of it. If you allow self hosting of the other code and include wiki.js for a local copy of your documentation then APGL might trigger.
Linking is meaning software linking, not referencing your own content displayed by the software. Think of it as the same difference between including using chunks of a paper in your own paper, or saying "as discussed in This Paper About This Thing (Him, Her, et. al, 2013)...". Or, as in this case the content is your's, referencing your own paper.
This potential confusion with the word "linking" (as natural languages are fairly dynamic beasts) is why people are sometimes overly fearful of AGPL and GPL (and often even LGPL) software.