Morebits: add documentation (wikimedia-gadgets#481)

* morebits: improve documentation of wiki.api, string, array classes

* morebits: wiki.page: generate jsdoc comments on basis of existing doccomment

* morebits: wiki.page: prune the doc comment at the top to show just the highlights, rearranged functions to a more logical order, to roughly match with the order in the original doc comment

* morebits: use jsdocs on wiki.preview; minor changes to wiki.api

* morebits: add jsdocs for wikitext.page + minor changes elsewhere